Population genetics and phenotypic variation are associated with habitat type in giant waterbugs (Abedus herberti)

Background/Question/Methods

Abedus herberti, a giant waterbug in the family Belostomatidae, is found in perennial streams throughout the Madrean Sky Islands of Arizona and Northern Mexico.  Two distinct morphological forms of Abedus herberti have been identified and are specific to two habitat types.  The small morph is significantly smaller, and is found in lowland desert springs (termed cienegas); and the large morph is found in higher elevation mountain streams.  Cienega habitats have higher annual mean temperature and greater thermal and hydrologic stability than upland stream habitats, which fluctuate in temperature and habitat size seasonally.  Previous population genetics research, using mitochondrial markers, did not present a clear picture of how these two distinct morphs are related to each other.  We are currently using a 2bRAD-sequencing approach to determine if small body size has arisen multiple times within A. herberti, or if small morphs share a common ancestor.  This method allows us to examine genetic loci throughout the genome, including more conserved regions. 

Results/Conclusions

Due to their patchy distribution on the landscape and poor dispersal ability, Abedus herberti populations are highly genetically-differentiated and most populations contain unique alleles.  Preliminary analyses indicate that the small morph populations do not form a monophyletic group relative to neighboring large morph populations, suggesting that differences in body size have arisen multiple times independently due to similar habitat conditions.  Higher temperatures and stable flow regimes typical of cienega habitats may select for smaller, faster maturing individuals which can reproduce year-round.  Large individuals, in contrast, may be better able to cope with the cold temperatures of the winter season and extreme habitat reductions of the summer season common in upland habitats.  Understanding how this species reacts to environmental changes is critical, as at least one of the sampled populations (and likely two) has recently become extinct, and the low levels of gene flow among populations make recolonization events unlikely.