Disassortative mating in replicated secondary contact experiments in nature

Ecological speciation is now regarded as one of the primary processes by which new species are generated. Adaptive divergence in allopatry begins this process, but it is often unclear when and how mechanisms that promote reproductive isolation, such as assortative mating and selection against hybrids, evolve. Here, we test for evidence of these mechanisms across replicated secondary contact experiments in natural settings. We introduced four to eight allopatric populations of threespine stickleback ( Gasterosteus aculeatus ), in both single-ecotype and mixed-ecotype treatments, into nine natural lakes, after which we inferred mating patterns by genotyping the resulting F1 generation. Contrary to expectations from the literature, we found no evidence of assortative mating or partial reproductive isolation among the introduced source populations. Instead, we detected evidence of disassortative mating by source population in three lakes and some evidence of disassortative mating by ecotype in one lake. These mating patterns were both context-dependent and population-dependent, varying substantially across lakes receiving the same source populations, and with some source populations generally displaying greater tendencies for disassortment. The absence of positive assortative mating in any replicate demonstrates that adaptive divergence in allopatry alone might be insufficient to generateassortative mating in many cases, while the possibility of disassortative mating in these contexts poses an additional hurdle on the path toward speciation.