Strong 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 on the
possibility that T. californicus has polygenic sex determination
using an incomplete diallel crossing design and the “animal model” 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. 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.