Abstract
Genetic incompatibilities contribute to reproductive isolation between many diverging populations, but it is still unclear to what extent they play a role if divergence happens with gene flow. In contact zones between the "Crab" and "Wave" ecotypes of the snail Littorina saxatilis, divergent selection forms strong barriers to gene flow, while the role of post‐zygotic barriers due to selection against hybrids remains unclear. High embryo abortion rates in this species could indicate the presence of such barriers. Post‐zygotic barriers might include genetic incompatibilities (e.g. Dobzhansky–Muller incompatibilities) but also maladaptation, both expected to be most pronounced in contact zones. In addition, embryo abortion might reflect physiological stress on females and embryos independent of any genetic stress. We examined all embryos of >500 females sampled outside and inside contact zones of three populations in Sweden. Females' clutch size ranged from 0 to 1,011 embryos (mean 130 ± 123), and abortion rates varied between 0% and 100% (mean 12%). We described female genotypes by using a hybrid index based on hundreds of SNPs differentiated between ecotypes with which we characterized female genotypes. We also calculated female SNP heterozygosity and inversion karyotype. Clutch size did not vary with female hybrid index, and abortion rates were only weakly related to hybrid index in two sites but not at all in a third site. No additional variation in abortion rate was explained by female SNP heterozygosity, but increased female inversion heterozygosity added slightly to increased abortion. Our results show only weak and probably biologically insignificant post‐zygotic barriers contributing to ecotype divergence, and the high and variable abortion rates were marginally, if at all, explained by hybrid index of females.
Highlights
Westram et a and these might have generated additiona opportunities for DMIs to evo ve as by products of divergent se ection
| Characterizing fema e hybrid status the owest record on the most wave exposed rocks The embryos are ike y to be affected by physio ogica stress for examp e extreme temperatures when the fema e is emerged above seawater
Norma and abortive embryos of four different deve opmenta stages preve iger before deve opment of swimming organs ve a ve iger with ve a post ve iger after ve a is shed and hatch ing ready to eave the brood pouch Abortive embryos have various types of severe deve opmenta abnorma ities and are presumab y not ab e to survive outside the embryo pouch The bar is 0.5 mm Genotyping of fema e L. saxatilis was primari y used to identify of abortive embryos as a proportion of the tota number of embryos fema e hybrid status Important y ear ier resu ts indicate
Summary
A though severa ecotypes of Littorina saxatilis exist Reid this arrangements end up in the same heterokaryotype individua which study focused on Crab and Wave ecotypes that are adapted to spewi most frequent y happen in the hybrid zone incompatibi ities wi cific rocky shore microhabitats Johannesson be expressed a
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