Variance components of sex determination in the copepod Tigriopus californicus estimated from a pedigree analysis

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.