Morphological and mitochondrial DNA variation in a hybrid zone between short-tailed shrews (Blarina) in Nebraska

Abstract

The parapatric boundary separating Blarina brevicauda and B. hylophaga in Nebraska was analyzed to quantify hybridization. Multivariate analyses of morphological data from 88 reference animals and 715 shrews from two transects across the boundary found the species to be distinct; 49 animals, however, were morphologically intermediate and could not be identified to species. Those individuals were trapped at locations clustered near the line of contact, suggesting that they were hybrids. Hybrids of trappable age apparently do not suffer reduced survival rates but appear to be reproductively less fit than nonhybrids. The small number of hybrids found, the abrupt nature of the boundary, and the strong similarity of clines for different characters within transects suggests genetic exchange between the species is limited. Genetic analyses were conducted on 190 shrews from the boundary and 61 reference animals. Restriction-site analysis of the mitochondrial cytochrome b gene generated diagnostic patterns for the reference samples with two separate enzymes. From the boundary, 17 hybrids were found, recognizable by conflicting morphological and mitochondrial characteristics. Those individuals were not morphologically intermediate and probably represented second generation or back-cross hybrids, indicating F1 hybrids are not sterile. Data also indicate that interspecific mating involves females of both species. Overall, these analyses suggest that a lack of assortative mating, combined with selection against hybrids, may act to limit sympatry between these species. Given the number of parapatric boundaries present in Blarina, the fossil-based evolutionary history proposed, and the morphological differences of extant species, this group might represent a model system for hybrid zone research.