knitr::opts_chunk$set( collapse = TRUE, comment = "#>" )
Martijn van de Pol, Lyanne Brouwer and Andrew Cockburn - 11 August 2020
R package hiphop is a method for parentage assignment using bi-allelic genetic markers like SNPs (Single Nucleotide Polymorphism). It has widespread application (paternity and maternity assignment in a variety of mating systems) and outperforms conventional methods where closely related individuals occur in the pool of possible parents. The method is based on simple exclusion logic by comparing the genotypes of offspring with those of potentials parents using the HIPHOP and HOT test; the rationale behind this paternity assignment method is described in detail in Cockburn et al. (for full reference use citation("hiphop")). Here we explain how it can be used with worked examples based on the data set that are used in the above paper and are included in the hiphop package.
The basic idea behind the HIPHOP parentage assignment is that we can compare the genotypes of any combination of offspring with potentials dams and sires (i.e. genetic mother or fathers) by comparing the exclusion scores of these individuals at bi-allelic markers. It elaborates on a prior exclusion method based on the Homozygous Opposite Test (HOT; Huisman 2017; https://doi.org/10.1111/1755-0998.12665). HOT compares the genotype of an offspring with a potential parent: a mismatch is scored when both the offspring and parent are homozygous, but for different alleles. Cockburn et al. suggested an additional -and as we will later see more informative- exclusion criterion called HIPHOP (Homozygous Identical Parents, Heterozygous Offspring are Precluded). This test compares the genotype of an offspring with both potential parents: a mismatch is scored when the offspring is heterozygous and both parents are homozygous for the same allele. Furthermore, although the HOT test was originally designed to compare the genotypes of offspring with one parent (a dyad), it can easily be extended to also compare offspring to both parents (a triad, similar to the HIPHOP test that also compares triads): a mismatch is scored when the offspring is homozygous and at least one of the parents is homozygous for the opposite allele (i.e. the HOT.parents test). The HOT and HIPHOP scores of triads can be combined to provide information about the total of mismatches and is proposed as an improved exclusion criterion for parentage assignment, particularly when distinguishing between closely related potential parents.
Below mismatch table shows all possible combinations of bi-allelic genotypes of offspring-dam-sire combinations for a single locus and which combinations of genotypes are indicative of a mismatch (the R code also illustrates the basic calculations). The total number of mismatches (also called the 'HOTHIPHOP.parents' mismatch score) is the sum of the HOT scores of the offspring with the parents (HOT.parents) and the HIPHOP score with the potential parents (offspring-dam-sire triad). Mismatches may occur because (a) the potential dam and/or sire is not the true genetic parent, (b) genotyping errors, and (c) mutations. We are aiming to exclude potential parents by determining if they have more mismatches than can be expected due to genotyping errors and mutation, and thereby identify (i) the true genetic parents and (ii) detect situations where one (or both) of the true parents is not sampled, in which case we (iii) want to identify which parent can still be correctly assigned.
genotyp<-c("AA", "Aa", "aa") gentable.names<-c("offspring","dam","sire","hot.dam","hot.sire","hot.parents", "hiphop","hothiphop.parents") gentable<-as.data.frame(array(NA,dim = c(length(genotyp)^3,length(gentable.names)), dimnames=list(NULL,gentable.names))) gentable$offspring<-rep(genotyp, each=length(genotyp)*length(genotyp)) gentable$dam<-rep(genotyp, length(genotyp)*length(genotyp)) gentable$sire<-rep(genotyp, length(genotyp), each=length(genotyp)) # the HOT test for the offspring-dam dyad gentable$hot.dam<-ifelse( (gentable$offspring=="AA" & gentable$dam=="aa") | (gentable$offspring=="aa" & gentable$dam=="AA"), 1,0) # the HOT test for the offspring-sire dyad gentable$hot.sire<-ifelse( (gentable$offspring=="AA" & gentable$sire=="aa") | (gentable$offspring=="aa" & gentable$sire=="AA"), 1,0) # the HOT test for the offspring-dam-sire triad gentable$hot.parents<-ifelse(((gentable$offspring=="AA" & (gentable$sire=="aa" | gentable$dam=="aa")) | (gentable$offspring=="aa" & (gentable$sire=="AA" | gentable$dam=="AA"))),1,0) # the hiphop for the dam-sire combination gentable$hiphop<-ifelse( (gentable$offspring=="Aa" & gentable$sire=="aa" & gentable$dam=="aa") | (gentable$offspring=="Aa" & gentable$sire=="AA" & gentable$dam=="AA"), 1,0) gentable$hothiphop.parents<-gentable$hot.parents+gentable$hiphop print(gentable)
Parentage analysis typically comes in two forms:
In the conventional case of parentage analysis there is usually contextual information that provides evidence about the identity of one of the genetic parents. The most common case will be where a female has been observed to be involved in reproductive tasks (built a nest, laid or incubated the eggs, lactated young). For the male parent we typically cannot make this assumption, because contextual evidence for paternity is usually less obvious and promiscuity means that paternity is often uncertain and needs to assessed (though in some species with sex role reversal, it may be possible to condition on the social father and focus on maternity assignment).
In other systems we have no or little contextual information about either of the social parents, in which case we can make no a priori assumptions about the genetic parents. In such cases the aim is to assign both the genetic mother (dam) and genetic father (sire) and perform a parent pair analysis. Such cases occur when: (a) Communal spawning: some animals with external fertilisation that breed in aggregations so that it is impossible to know which female laid eggs (e.g. frogs in leks, some fish) and which male fertilized the eggs, as there are no permanent social groups. (b) Polygynandry: multiple females and males breed and deposit their young in a common burrrow or nest (e.g. acorn woodpeckers, Eurasian badgers), which may be difficult to observe so individual offspring cannot be associated with a single parent. There may also be extra-group mating (as has been argued for chimpanzees and Australian magpies).
R package hiphop includes three functions and an example dataset consisting of two input dataframes based on five cohorts (2014-2018) of superb fairy wrens living in Canberra, Australia.
We first load the library.
The superb fairy wren case reflects the first case described above, as there is a social pair, in which previous evidence has shown that the female that incubates the eggs (the social mother) is always the dam. However, the social male is generally not the sire, as the rate of extra-pair paternity is among the highest recorded in bird species. Furthermore, besides the male that is pair bonded to the social mother (the social father), each group contains supernumerary males that sometimes also gain paternity (there is within- and extra-group paternity). Finally, superb fairy wrens have very limited dispersal, almost all males live their entire life on their natal or a neighouring territory, and as a consequence paternity assignment thus faces the challenge of distinguishing between closely related potential sires (e.g. first order relatives).
In the next sections we will first use the example dataset to illustrate the workings of the functions in package hiphop. Next we will use superb fairy wrens to illustrate how to conduct single parent assignment of the sires. Finally, we will also use the same dataset to illustrate how to perform parent-pair analyses for the second case (by ignoring the fact that in superb fairy wrens we know the genetic mother a priori).
The dataset consist of two dataframes/files. The first input dataframe -called 'individuals'- consists of a list of 1153 offspring that we would like to assign parentage to and 1373 records of potential parents (252 adult males and 145 adult females). The file has 5 variables:
The first variable denotes the brood to which the individual belongs, the second variable is the identity of the individual, the third variable (type) denotes whether it is an offspring, adult male (i.e. a potential sire) or adult female (potential dam). The fifth column (social.parent) describes whether the individual is a social parent at the brood it is associated with (for females 1, if she incubated or build the nest; for males 1, if he was the dominant male pair bonded with the social mother) or not (0; supernumerary males). The final variable (year) is to separate cohorts to be analyzed. Note that parentage analyses is done typically done one cohort at a time, as the set of potential dams and sires will vary across years (as some individuals were not born yet or were already dead in some years).
Thus individuals may appear in multiple years and adults may also appear multiple times within a year if they were involved in multiple broods. For example, the above 10 records show two broods of the same breeding pair (982804-awRM & 899972-rnWR). Both broods produced three offspring, but the second brood had a supernumerary male (982045-BwgW) helping the parents.
It is important to realise that there will usually be potential parents living both within and outside the study area that will not be considered if just the adults associated with a brood are included in this file. Cockburn et al. showed that some young are sired by males living outside the study area, and in other species floaters might be important. A universal problem will be parents living within the study area that do not raise young to the point where DNA can be sampled. Ensuring that such adults are considered as parents can be achieved by giving them a dummy brood ID. As can be seen below, in our dataset we have added such males using brood values such as other353, etc.
In some mating systems it will not be possible to separate the young into broods and there is no parental care. In such cases all young and potential parents can be given the same brood ID (with brood potentially meaning day of sampling, frog pond, fish tank). We note that the brood ID is not used for calculating the exclusion scores, it is only used to qualify any potential parent as either within- or extra-group (i.e. extra-brood) parent.
The second input dataframe/file -called 'genotypes'- consists of a list of all genotyped individuals (offspring and potential parents) and their genotypes scored using bi-allelic markers. In the example dataset it comprises 1407 individuals scored at 1376 loci.
The column names are alphanumerics used to identify each SNP (Single Nucleotide Polymorphism) and the row names identify each individual. Values are specified as follows:
If an individual was not sampled and/or genotyped it should not be included in the genotypes dataset.
R package hiphop only contains 3 functions: inspect, hothiphop and topmatch.
The hothiphop function extracts the list of potential dams and potential sires for a specific cohort of offspring from the individuals dataset. For that cohort it will consider all adult females listed that year as potential dams and all adult males listed that year as potential sires. For reasons given above, we analyse one cohort/year at a time.
We thus first subset the individuals dataset to one cohort, here the 2018 cohort of superb fairy wrens.
Next we inspect the genotypes' summaries and identify any individual that has not been genotyped:
inspection<-inspect(ind=ind2018, gen=genotypes) head(inspection) inspection[which(inspection$sampled==0),]
The summary lists, in addition to some details of the individual, the proportion of loci for which the individual is homozygous, heterozygous or has missing data. We see that most individuals have very few missing loci data (mostly <1%), but that there is one social father (SOC018-fBab) that has not been sampled.
Next we run the mismatch analysis for all offspring-potential dam-potential sire combinations for the 2018 cohort using the hothiphop function. This function calculates the proportion of genetic mismatches according to the HIPHOP and HOT test.
In the 2018 cohort there are 149 offspring, 42 potential dams and 73 potential sires (each superb fairy wren brood has a social pair and 0-5 supernumerary adult males).
print(c(length(unique(ind2018$individual[which(ind2018$type=="offspring")])), length(unique(ind2018$individual[which(ind2018$type=="adult female")])), length(unique(ind2018$individual[which(ind2018$type=="adult male")]))))
This means that there are about half a million possible combinations of offspring-dam-sire (149x42x73), so the next code may take several minutes to run:
This output is large and contains all the required information to assign parentage. Basically it lists all possible offspring-potential.dam-potential.sire combinations and their mismatch scores (unsorted), the number of loci this was based on, and some relevant information about the social parents and potential dam and sires. For a detailed explanation see the help file of the function ('?hothiphop()')
However, due to the large number of combinations it can be difficult to find the salient information, so the results can be summarized into a more insightful format by extracting the top matches for each offspring using the 'topmatch' function. By default for each offspring the 'topmatch' returns the 3 top matches for male/female combinations according to the chosen ranking criteria, and additionally if they are not yet included, the results for the triad comprising the social parents.
best.hothiphop<-topmatch(x=combinations, ranking="hothiphop.parents") best.hothiphop[1:8,]
For offspring A59401-iOar (the first four lines of output) we can see that the triad A58110-BBW (dam) & A59058-zMyr (sire) has by far the fewest HOTHIPHOP.parents mismatches (37; HOTHIPHOP.parents = HOT score of the parents/triad + HIPHOP score of triad), much less than the second ranked dyad (109 mismatches; the same dam with another sire). The best ranked dam A58110-BBW is the social mother, while the best ranked sire A59058-zMyr is an extra-group parent (as it is not associated with the same brood as the offspring in the individuals dataset). In line 4 we can see that the social father also has many more mismatches (191), and is unlikely to be the sire. In rows 4-8 we see another offspring from the same brood, which in this case is sired by the same extra-group male A59058-zMyr (33 mismatches).
It is possible to set a threshold value that determines whether the dam, sire or triad should be accepted as parents (see later for determining a threshold).
best<-topmatch(x=combinations, ranking="hothiphop.parents", thres=62) best[9:12,c(1:9, 16,17)]
In our dataset, we found empirically that for this species and dataset (level of genotyping error) the true parents never had more than 62 HOTHIPHOP.parents mismatches (Cockburn et al.), and by setting the threshold to this value we can see that in the column 'below.threshold' all triads above (below) this value are labelled as 0 (1). If not specified, the default threshold value is a large value (99999) meaning that all triads are below the threshold.
If the threshold value is set to a value between 0 and 1 then the number of mismatches are recalculated to the proportion of mismatches (number of mismatches / number of loci that are not NA), which is a more accurate score if individuals differ strongly in the number of loci with missing data.
best<-topmatch(x=combinations, ranking="hothiphop.parents", thres=0.045) best[9:12,c(1:9, 16,17)]
Finally, if the threshold value is set to a value between 0 and -1 then the number of mismatches are recalculated to the proportion of mismatches, but in contrast to above example the mismatch scores are expressed as the number of mismatched divided by the number of loci at which the offspring is heterozygote (for the HIPHOP test) or divided by number of loci at which the offspring is homozygote (for the HOT test). Thus, by setting 'thres=-0.99' we aim to correct for the fact that heterozygous offspring will accumulate more HIPHOP and less HOT mismatches, all else being equal. For calculating whether triads are below.threshold, they will be compared to abs(thres).
best<-topmatch(x=combinations, ranking="hothiphop.parents", thres=-0.99) best[9:12,c(1:9, 16,17)]
The 'topmatch' function can rank matches via several criteria specified by the parameter 'ranking'. Options are: "hothiphop.parents" , "hiphop" 'hot.dam', 'hot.sire' as well as 'hothiphop.dam' and 'hothiphop.sire' (see ?topmatch() for explanation of how these criteria differ). For example:
best.hiphop<-topmatch(x=combinations, ranking="hiphop") best.hiphop[1:4,1:13]
At first glance, the HOTHIPHOP.parents scores may appear most informative as it sums all possible HIPHOP and HOT mismatches of the triad. However, the HOT test is disproportionately affected by genotyping error, and thus potentially HIPHOP can give cleaner comparisons in some cases than HOTHIPHOP.parents. As a result of above complexity (as well as other dependencies of HOT and HIPHOP scores on the rate of hetero- and homo-zygosity of the offspring and parents), the choice and best definition of exclusion criterion is not a trivial matter. Therefore, we refer to Cockburn et al. for a more in depth discussion of the pros and cons of different scoring criteria and how they can differ in their conclusions in parentage assignment.
We note that when ranking on the HOT test scores of dyads (e.g. on HOT.dam as shown below), that the top ranked triads will always contain the same dams (but with different sires), as the HOT.dam score -in contrast to the HIPHOP and HOT.parents score- is not calculated on a triad, but on an offspring-parent dyad and thus does not depend on the genotype of the other parent.
best.hot.dam<-topmatch(x=combinations, ranking="hot.dam") best.hot.dam[1:3,1:13]
To compare the HOT scores of dams it is necessary to specify that you are only interested in one record per dam, which can be achieved by setting the unique argument:
best.hot.dam<-topmatch(x=combinations, ranking=c("hot.dam","hiphop"), unique="dam") best.hot.dam[1:4,1:13]
Now we get the top 3 unique dams ranked according to hot.dam score (as well as the social mother, if not in the top 3), and because we have also included a second ranking argument "hiphop" the dams are shown with the sires with whom they have the lowest hiphop score.
Furthermore, we can use the 'condition' parameter to also condition the parentage analysis on prior contextual knowledge of the parentage of one parent. In most species prior knowledge will relate to the social mother, as the fact that she is lactating, laid or incubated the eggs may provide such information (assuming e.g. egg dumping does not occur).
best<-topmatch(x=combinations, ranking="hothiphop.parents", condition="mother") best[1:4,1:13]
Now we can see that only the female that was classified as social parent is considered as dam, while all males are considered as potential sire. The conditioning on one parent will facilitate the assignment of the parentage of the other parent, unless the unconditioned parent is incorrect (see later on how to verify this assumption). For some specific species (e.g. with sex role reversal) one may instead want to condition on the social father instead.
Finally, it is possible to increase the number of combinations being ranked, by changing the value of the top parameter (the default is top=3, which shows the top 3 combinations plus the social parents). For example:
best<-topmatch(x=combinations, ranking="hothiphop.parents", top=5) best[1:6,1:7]
Previously we identified two types of cases in which paternity analysis can be conducted. Next we provide recommendations on how to proceed in such cases:
The classical procedure is to first verify the assumption that the mother is known (using the HOT.dam criteria), and then condition on the social mother to determine the most plausible sire, which we can do with either the HOTHIPHOP.parents or HIPHOP criteria. Which of these two ranking score is most useful may depend on the amount of genotyping error and rates of hetero- and homo-zygosity in the population. The choice is thus largely an empirical question and it is useful to consider both scores (for more discussion see Cockburn et al.).
Thus we first compare the dams with the lowest HOT.dam scores to verify whether there were any cases in which the social mother has more mismatches than expected due to genotyping error.
best.dams<-topmatch(x=combinations, ranking=c("hot.dam","hiphop"), unique="dam") best.dams[1:8,1:13] which(best.dams$rank==1 & best.dams$dam.type!="social parent")
We see that in this cohort there were no cases where the social mother was not the first ranked dam (i.e. with lowest hot.dam score).
best.triad<-best.dams[which(best.dams$rank==1),] best.triad<-best.triad[!duplicated(best.triad[c("offspring","dam")]),] #we remove the duplicates, because if the dam with the lowest hot.dam score is the social mother she can be listed twice: once with the social father and one with the sire that gives the lowest HIPHOP score plot(best.triad$hot.dam,best.dams$hot.dam[which(best.dams$rank==2)], xlim=c(0,100), ylim=c(0,100), xlab="Hot.dam score 1st ranked dams (here the social mothers)", ylab="Hot.dam score 2nd ranked dams", main="149 offspring of 2018 cohort") abline(0,1)
We can also see that these first ranked dams, which we already confirmed were the social mothers, had lower HOT.dam scores than the second ranked dams, but we note that there is one case in which the 2nd ranked dam has a HOT.dam score of 29 mismatches, which is within the range of HOT.dam scores of social mothers. In such situations, which typically involves close relatives of the social mother, the HOTHIPHOP.parents or HIPHOP score provides additional information to distinguish the true dam, but only if the true sire is sampled.
This output shows that although the 2nd ranked dam A30173-ngNA had a HOT.dam score close to that from the social mother 982682-rgMM (29 vs. 20), from the high HIPHOP score (20) it is clear she was not the genetic mother, but that the social mother was indeed the genetic mother (as she had 0 HIPHOP mismatches, which also indicates the true sire was sampled, as the HIPHOP score is calculated on a triad). Please note that first line of above ouput shows the social mother with the sire with whom she has the lowest HIPHOP score (in this case an extra-group sire), while the second line shows the social mother with the social father, which resulted in a many HIPHOP mismatches (96)
A more visual way of determining whether the social mother is also the genetic mother can be done by combining the above two steps in one plot. This approach is best illustrated on all cohorts. Below we provide the code to do this on all cohorts, but users can only run the 2018 cohort if this takes too long (if so, start at line with plot-function).
# first run the hothiphop on all cohorts comb.18<-hothiphop(ind=subset(individuals,individuals$year==2018), gen=genotypes) comb.17<-hothiphop(ind=subset(individuals,individuals$year==2017), gen=genotypes) comb.16<-hothiphop(ind=subset(individuals,individuals$year==2016), gen=genotypes) comb.15<-hothiphop(ind=subset(individuals,individuals$year==2015), gen=genotypes) comb.14<-hothiphop(ind=subset(individuals,individuals$year==2014), gen=genotypes) # then run topmatch function on all cohorts best.18<-topmatch(x=comb.18, ranking=c("hot.dam","hiphop"), unique="dam") best.17<-topmatch(x=comb.17, ranking=c("hot.dam","hiphop"), unique="dam") best.16<-topmatch(x=comb.16, ranking=c("hot.dam","hiphop"), unique="dam") best.15<-topmatch(x=comb.15, ranking=c("hot.dam","hiphop"), unique="dam") best.14<-topmatch(x=comb.14, ranking=c("hot.dam","hiphop"), unique="dam") #combine results combinations.all<-do.call("rbind", list(comb.18, comb.17, comb.16,comb.15, comb.14)) best.dams<-do.call("rbind", list(best.18, best.17, best.16,best.15, best.14)) #remove the cases where the social mother was the first ranked dam, as then she is listed twice with rank 1 best.triad<-best.dams[which(best.dams$rank==1),] best.triad<-best.triad[!duplicated(best.triad[c("offspring","dam")]),] # plot the data plot(best.triad$hot.dam ,best.triad$hiphop, col = "red", xlab="HOT.dam", ylab="HIPHOP", xlim=c(0,100), ylim=c(0,120)) points(best.dams$hot.dam[which(best.dams$rank==2 & best.dams$dam.type!="social parent")],best.dams$hiphop[which(best.dams$rank==2 & best.dams$dam.type!="social parent")], col = "blue") points(best.dams$hot.dam[which(best.dams$rank==1 & best.dams$dam.type!="social parent")],best.dams$hiphop[which(best.dams$rank==1 & best.dams$dam.type!="social parent")], col = "orange", pch = 19) points(best.dams$hot.dam[which(best.dams$rank==2 & best.dams$dam.type=="social parent")],best.dams$hiphop[which(best.dams$rank==2 & best.dams$dam.type=="social parent")], col = "green", pch = 19) legend("bottomright",c("1st ranked social mother", "1st ranked not social mother", "2nd ranked social mother", "2nd ranked not social mother"), fill = c("red", "orange", "green", "blue"))
Here we plotted the HOT.dam and HIPHOP score of the first and second ranked dam (ranked according to HOT.dam score, with the best matching sire according to the HIPHOP score) against their HIPHOP score. We also use different colors depending on whether they are the social mother or not. We see that that first ranked social mothers typically have both low HOT.dam and HIPHOP scores, while the occasional second ranked female with low HOT.dam scores (<38 mismatches, within the range of social mothers) always had >16 HIPHOP mismatches. Furthermore, we can also see that there are ~25 1st ranked social mothers with low HOT.dam scores but high HIPHOP scores, which we will discuss in the next section are situations where the sire was not sampled. Finally, even in the rare case when the social mother is not the first ranked dam on the HOT score (green dots; 5 out of 1153 offspring), she is still clearly identifiable as the genetic mother due to the low HIPHOP score (and non-social mothers that are ranked 1st on HOT.dam score always have high HIPHOP scores).
Above figure emphasizes that the HOT test in itself is thus insufficient to distinguish parentage among closely related individuals (as there were ~55 blue 2nd ranked triads of non-social mothers with HOT.dam scores similar to social mothers), but the HIPHOP (and HOTHIPHOP.parents) score is capable of doing so.
Once it is confirmed that the assumption that the social mother is always the dam is highly plausible, then we can condition on the mother and rank the sires, using either HIPHOP or HOTHIPHOP.parents scores.
best.parents.hiphop<-topmatch(x=combinations, condition="mother", ranking="hothiphop.parents")
plot(best.parents.hiphop$hiphop[which(best.parents.hiphop$rank==1)],best.parents.hiphop$hothiphop.parents[which(best.parents.hiphop$rank==1)], col = "red", xlab="HIPHOP", ylab="HOTHIPHOP.parents", main="149 offspring of 2018 cohort") points(best.parents.hiphop$hiphop[which(best.parents.hiphop$rank==2)],best.parents.hiphop$hothiphop.parents[which(best.parents.hiphop$rank==2)], col = "blue") legend("topleft",c("1st ranked", "2nd ranked"), fill = c("red", "blue"))
We can see that in many cases the best ranked triad has 0 or 1 mismatches for the HIPHOP test score, suggesting that the true sire is identified in this triad. In this case both the HIPHOP and HOTHIPHOP.parents score distinguish well between the true sires (1st ranked sires) and others (2nd ranked sires). We note that in the 2018 data under consideration, there were five outliers with high mismatch rates (the five red dots in the blue cloud).
best.parents.hiphop[which(best.parents.hiphop$rank==1 & best.parents.hiphop$hiphop>20),c(1:13,22,23)]
These five cases are from two broods. Three chicks from sNab_7237 had HIPHOP mismatches ranging from 66 to 85. The nest was close to the edge of the study area, and we believe it was sired by a foreign (unsampled) male. The second brood (fBab_7322) of two chicks also had high mismatches (97 & 119); Cockburn et al. discuss this case in detail. The nest was close to the edge of the study area but the social father was unsampled (see 0 in column social.father.sampled), so in this case we cannot ascribe paternity, and also cannot say whether the chicks were within-pair or not.
To illustrate the procedure for parent pair assignment (i.e. none of the genetic parents are known from contextual information), we again use the superb fairy wren dataset, but will now assume that we have no a priori information about the genetic mother and will focus on both maternity and paternity assignment. The superb fairy wren dataset only consist of offspring for which the true dam was sampled, which is unrealistic for many other study systems. To illustrate how to deal with situations of less complete sampling (also for the males), we removed 20% of the social mothers and 20% of the males from the individuals dataset.
ff<-which(individuals$type=="adult female") individuals.rem<-individuals[-ff[seq(5,length(ff),5)],] #this removes every fifth female in the individuals dataframe mm<-which(individuals.rem$type=="adult male") individuals.rem<-individuals.rem[-mm[seq(5,length(mm),5)],] #this removes every fifth male in the individuals dataframe
When both social parents are unknown, the aims is to assign both maternity and paternity. The first step is to determine whether we get a clearly separated bi-modal distribution of HOTHIPHOP.parents or HIPHOP scores between the first ranked and higher ranked parents. Ideally this is done on a large dataset, so on all cohorts available (to skip the long run time, one could instead subset the individuals.rem dataframe below to the year 2018).
combinations.rem<-hothiphop(ind=individuals.rem, gen=genotypes) best.hhhh<-topmatch(x=combinations.rem, ranking="hothiphop.parents") h1 <- hist(best.hhhh[which(best.hhhh$rank==1), "hothiphop.parents"], breaks = 100) h2 <- hist(best.hhhh[which(best.hhhh$rank==2), "hothiphop.parents"], breaks = 50) plot(h2, col = "blue", xlim = c(0, 200), xlab="HOTHIPHOP.parents", main=NULL) plot(h1, col = "red", xlim = c(0, 200), add = T) legend("topright",c("1st ranked", "2nd ranked"), fill = c("red", "blue"))
As case 2 concerns situations where the social parents are unknown, we have no information a priori whether the 1st ranked individuals are indeed the true dam-sire combinations, or whether due to genotyping error some of the true combinations ended up as 2nd or higher ranked. However from both histograms it is clear there is a bi-modal distribution, and in such situations we can make plausible assumptions that if only first ranked individuals are in the left peak that these are likely only true parents. Specifically, we see that there is a red peak at the left with low <12 HIPHOP (or <64 HOTHIPHOP.parents in left histogram) mismatch scores, which likely reflect the true dam-sire triads with mismatches due to genotyping error and mutations. In addition we have a blue peak with higher scores that are likely to reflect incorrect combinations, as these combinations have (much) higher mismatch scores (due to incompatible genotypes as well as genotyping error and mutations).
There are quite some red bars (1st ranked triads) within the peak of blue bars (2nd ranked triads). Based on above rationale it follows that these are incorrect 1st ranked combinations that may reflect cases where the true sire (and/or dam) was not sampled (in fact most of these are the offspring for which we removed the social mothers and sires at the start of this section). For these combinations we could look at the HOT.dam and HOT.sire scores to determine whether the sire or dam was likely to be unsampled (or both).
plot(best.hhhh[which(best.hhhh$rank==1 & best.hhhh$hiphop<12),"hot.dam"] ,best.hhhh[which(best.hhhh$rank==1 & best.hhhh$hiphop<12),"hot.sire"], col ="orange" , xlab="HOT.dam", ylab="HOT.sire", xlim=c(0,100), ylim=c(0,100), pch = 19, main="1st ranked triads (on HIPHOP) 2018 cohort") points(best.hhhh[which(best.hhhh$rank==1 & best.hhhh$hiphop>=12),"hot.dam"] ,best.hhhh[which(best.hhhh$rank==1 & best.hhhh$hiphop>=12),"hot.sire"], col ="purple", pch = 19 ) abline(v=37, h=35, col="green") legend("topright",c("HIPHOP < 12", "HIPHOP >= 12"), fill = c("orange", "purple"))
We have plotted the first ranked triads (according to HIPHOP score) for all 1153 offspring 2014-2018 cohorts. Based on above histograms, triads with HIPHOP scores lower than 12 (orange) are presumed to be the true parents, while triads with higher HIPHOP scores are assumed to be cases where at least one of the true parents was not sampled. From this figure we can see that true triads never have HOT.dam and HOT.sire scores higher than 37 and 35 respectively (see green lines in figure above), and we can use this information to identify which parent was unsampled.
We end by noting that the parent-pair method we described above could also be used for the single parent assignment case for this dataset, but that in contrast to the single parent assignment approach described earlier it does not require one to look at HOT.dam scores:
The aim of the HIPHOP package/ approach is to exclude potential parents by determining if they have more mismatches than can be expected due to genotyping errors and mutation, and thereby identify (i) the true genetic parents and (ii) detect situations where one (or both) of the true parents is not sampled, in which case we (iii) want to identify which parent can still be correctly assigned. In this vignette we showed that both single parent and parent pair assignment based on HIPHOP or HOTHIPHOP.parents can assign genetic parents correctly, while this is not possible using only the HOT test. We can also identify offspring for which parent(s) were unsampled, but in this dataset we cannot with complete certainty identify a parent correctly if the other parent was not sampled (though if the sire (or dam) has a HOT.sire (HOT.dam) score in the range of what is typical for true sires (dams), we still have high certainty, but we just cannot exclude that a closely related unsampled parent may exist that was the true sire (or dam)). Assigning parents if one of the parents is not sampled, may be easier to do (i) in species that do not live among close relatives (superb fairy wrens have strong spatial genetic clustering due to their extreme male philopatry), (ii) if more loci are available for scoring, or (iii) if genotyping error is reduced, as the latter two may separate the HOT scores of true sires (or dams) from closely related sires (or dams) more clearly.
Please post all queries to the R-hiphop google group at https://groups.google.com/d/forum/r-hiphop
The hothiphop function checks all possible offspring-potential.dam-potential.sire combinations. If the number of possible dams, sires and/or offspring is very large the number of combinations will become very high, which may lead to memory problems in R. In such cases a workaround is to limit the number of offspring in the individuals dataset to a smaller number (but still include all potential dams and sires). For example, one could include the first 10 offspring in the individuals file, run the hothiphop function, next run the hothiphop function for the next 10 offspring, and append the results to the results from the previous set (parallelization is another option).
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