DOI: 10.5061/dryad.mpg4f4r4k
title: Variance components of sex determination in the copepod Tigriopus californicus estimated from a pedigree analysis

description: Extensive theory exists regarding population sex ratio evolution that
 predicts equal sex ratio (when parental investment is equal). In most
 animals, sex chromosomes determine the sex of offspring, and this fixed
 genotype for sex has made theory difficult to test since genotypic
 variance for the trait (sex) is lacking. It has long been argued that the
 genotype has become fixed in most animals due to the strong selection for
 equal sex ratios. The marine copepod Tigriopus californicus has no sex
 chromosomes, multiple genes affecting female brood sex ratio and a brood
 sex ratio that responds to selection. The species thus provides an
 opportune system in which to test established sex ratio theory. In this
 paper, we further our exploration of polygenic sex determination in T.
 californicus using an incomplete diallel crossing design for analysis of
 the variance components of sex determination in the species. Our data
 confirm the presence of extra-binomial variance for sex, further
 confirming that sex is not determined through simple Mendelian trait
 inheritance. In addition, our crosses and backcrosses of isofemale lines
 selected for biased brood sex ratios show intermediate phenotypic means,
 as expected if sex is a threshold trait determined by an underlying
 “liability” trait controlled by many genes of small effects. Furthermore,
 crosses between families from the same selection line had similar
 increases in phenotypic variance as crosses between families from
 different selection lines, suggesting families from artificial selection
 lines responded to selection pressure through different underlying genetic
 bases. Finally, we estimate heritability of an individual to be male or
 female on the observed binary scale as 0.09 (95% CI: 0.034-0.14). This
 work furthers our accumulating evidence for polygenic sex determination in
 T. californicus laying the foundation for this as a model species in
 future studies of sex ratio evolution theory.
description: <em>Tigriopus californicus</em> are tidal pool
 copepods with unusual variance in brood sex ratio. Field-caught
 individuals were used to create two artificial selection lines using
 truncation selection to select for highly male-biased broods in one line
 and highly female-biased broods in another line. After six generations,
 these lines were used to do a diallel cross of families within and between
 lines, generating an F1 generation, followed by backcrosses to the
 parental lines. Crosses were done using eight families from each parental
 line in two blocks of crosses between four families for F1 generation. The
 sex of all individuals from each brood in the crosses was observed and a
 pedigree used to an "Animal Model" analysis to estimate the
 quantitative genetic variance in sex of individuals in a generalized
 linear model using Bayesian analysis. Sex in this model was treated as a
 threshold trait - a binary phenotype driven by an underlying
 (unobservable) continuous 'liability' phenotype of many genes of
 small effect, with sex determined based on some threshold value in this
 underlying trait. This analysis allowed estimation of heritability of sex
 in <em>Tigriopus californicus,</em> a species with polygenic
 sex determination. 
description: Data are provided in comma-delimited text files.
 Analysis code are provided for use with R.