Summary
The potential role of alternative reproductive tactics in circumventing premating isolating mechanisms and driving hybridization between species has long been recognized, but to date there is little empirical support from natural systems. Hybridization occurs between bluegill (Lepomis macrochirus) and pumpkinseed sunfish (Lepomis gibbosus) and it is known to be asymmetrical (male bluegill × female pumpkinseed). Here, we test whether this pattern is driven by a recognition failure by pumpkinseed females or by an alternative cuckolder reproductive tactic in bluegill males. Using genetic parentage data, we found that bluegill cuckolders fathered 24.9% of the larvae in bluegill nests, but no evidence that pumpkinseed females spawned in bluegill nests. Pumpkinseed cuckolders fathered 8.7% of the larvae in pumpkinseed nests, whereas bluegill cuckolders fathered 13.6% of the larvae in those nests. Bluegill cuckolders thus frequently spawn in pumpkinseed nests and are responsible for the asymmetrical hybridization between the species. We discuss the evolutionary consequences of interactions between bluegill and pumpkinseed and the role of alternative reproductive tactics in adaptation and introgression.
Methodology
Each individual was genotyped at three microsatellite loci: RB7, LMAR10 and either RB20 or LMAR14 (for details of these loci see [9,10]). RB20 was used for the parentage analysis of the 2005 bluegill samples, whereas LMAR14 was used for the interspecies allele frequency comparisons and the parentage analysis of the 2003 and 2010 pumpkinseed samples.
We identified microsatellite loci that were informative for species identification by comparing the distribution of allele sizes in parental males from each species. We found that LMAR10 and LMAR14 were each partially informative for species identification, and in combination would correctly identify 100% of pure bluegill, 100% of pure pumpkinseed and 97.6% of hybrids (detailed calculations are available in the electronic supplementary material). These loci were then used to assign each larva as a bluegill, pumpkinseed or hybrid. Additionally, for a small number of larvae with ambiguous ancestry (n = 9), we used a mitochondrial restriction assay (described in the electronic supplementary material) to determine whether the female parent was a bluegill or pumpkinseed.
For each nest, we calculated the proportion of paternity that could be attributed to the parental male and to cuckolders of each species. Hybrid larvae have low probability of being genetically compatible with a parental male by chance, so paternity for these larvae was assigned based on simple exclusion. For the remaining larvae, the share of paternity for the parental male and for cuckolders of his species was determined using the two-sex paternity model [11]. In pumpkinseed nests, the paternity of pumpkinseed parental males, pumpkinseed cuckolders and bluegill cuckolders did not differ significantly between 2003 and 2010 (t-tests, all p > 0.05), and data from these years were thus combined in our analyses.