Postzygotic isolation involves strong mitochondrial and sex-specific effects in Tigriopus californicus,a species lacking heteromorphic sex chromosomes

Detailed studies of the genetics of speciation have focused on a few model systems,particularly Drosophila. The copepod Tigriopus californicus offers analternative that differs from standard animal models in that it lacks heteromorphicchromosomes (instead, sex determination is polygenic) and has reduced opportunities forsexual conflict, because females mate only once. Quantitative trait loci (QTL) mapping wasconducted on reciprocal F₂ hybrids between two strongly differentiatedpopulations, using a saturated linkage map spanning all 12 autosomes and themitochondrion. By comparing sexes, a possible sex ratio distorter was found but no sexchromosomes. Although studies of standard models often find an excess of hybrid malesterility factors, we found no QTL for sterility and multiple QTL for hybrid viability(indicated by non-Mendelian adult ratios) and other characters. Viability problems werefound to be stronger in males, but the usual explanations for weaker hybrid males (sexchromosomes, sensitivity of spermatogenesis, sexual selection) cannot fully account forthese male viability problems. Instead, higher metabolic rates may amplify deleteriouseffects in males. Although many studies of standard speciation models find the strongestgenetic incompatibilities to be nuclear–nuclear (specifically Xchromosome–autosome), we found the strongest deleterious interaction in this systemwas mito–nuclear. Consistent with the snowball theory of incompatibilityaccumulation, we found that trigenic interactions in this highly divergent cross weresubstantially more frequent (>6 × ) than digenic interactions. This alternativesystem thus allows important comparisons to studies of the genetics of reproductiveisolation in more standard model systems.