Microhabitat Population Structure within a Mummichog Hybrid Zone

The mummichog, Fundulus heteroclitus, is a small baitfish that inhabits estuaries along the east coast of North America, from Newfoundland, Canada to northern Florida, USA. Gametic, embryonic, morphological, and genetic differences occur between individuals occupying the northern and southern parts of this range. Such differences have lead to the recognition of two subspecies, with a hybrid zone located along coastal New Jersey, USA. Laboratory experiments indicate that “northern” fish have better survival in freshwater and a lower thermal tolerance than “southern” fish. Using individuals collected from a single marsh from the center of the hybrid zone we have conducted an analysis of fine-scale population genetic structure. The 3.2 kilometer long marsh was sub-sampled at 3 locations (n = 110) in June 2008 and 8 locations (n = 339) in November 2009. Samples were analyzed using subspecies-specific SNPs within lactate dehydrogenase and mitochondrial D-loop, as well as 9 microsatellite loci. Genetic data were combined with salinity and temperature data to examine correlations between genetic makeup and capture location. For the June 2008 samples, there was no significant relationship between capture location and mtDNA type, LDH genotype, or hybrid index generated from the microsatellite data, but there was a trend towards increasing frequency of northern mitotype and LDH genotype with increasing salinity and decreasing temperature. These results are consistent with predictions based on the lower temperature tolerance of northerns. Conversely, the hybrid index indicated a higher occurrence of northern individuals in the more freshwater end of the marsh when compared to the more saline mouth while the frequency of southern individuals was similar across all locations. This is consistent with predictions based on the northern subspecies’ higher freshwater tolerance. Because such a pattern may be even more pronounced without the confounding effects of temperature, we performed additional fine-scale sampling of this marsh in November 2009 when water temperature was less variable. In these samples there was no significant relationship between mtDNA type and location. However, there was a significant dependence of LDH genotype on capture location, with more individuals of the southern LDH type occurring in areas of lower salinity, the opposite of the predicted direction. These results conflict with those obtained from the hybrid index, which indicated a higher occurrence of northern individuals in the same region of lower salinity. We are currently genotyping these fish at 37 additional SNPs to resolve these conflicting results.