Hybrid female mate choice as a species isolating mechanism: environment matters.

A fundamental goal of biology is to understand how new species arise and are maintained. Female mate choice is potentially critical to the speciation process: mate choice can prevent hybridization and thereby generate reproductive isolation between potentially interbreeding groups. Yet, in systems where hybridization occurs, mate choice by hybrid females might also play a key role in reproductive isolation by affecting hybrid fitness and contributing to patterns of gene flow between species. We evaluated whether hybrid mate choice behaviour could serve as such an isolating mechanism using spadefoot toad hybrids of Spea multiplicata and Spea bombifrons. We assessed the mate preferences of female hybrid spadefoot toads for sterile hybrid males vs. pure-species males in two alternative habitat types in which spadefoots breed: deep or shallow water. We found that, in deep water, hybrid females preferred the calls of sterile hybrid males to those of S. multiplicata males. Thus, maladaptive hybrid mate preferences could serve as an isolating mechanism. However, in shallow water, the preference for hybrid male calls was not expressed. Moreover, hybrid females did not prefer hybrid calls to those of S. bombifrons in either environment. Because hybrid female mate choice was context-dependent, its efficacy as a reproductive isolating mechanism will depend on both the environment in which females choose their mates as well as the relative frequencies of males in a given population. Thus, reproductive isolation between species, as well as habitat specific patterns of gene flow between species, might depend critically on the nature of hybrid mate preferences and the way in which they vary across environments.

Development and characterization of nine polymorphic microsatellite markers for Mexican spadefoot toads (Spea multiplicata) with cross-amplification in Plains spadefoot toads (S. bombifrons).

We developed nine polymorphic microsatellite markers for the Mexican spadefoot toad, Spea multiplicata. Allele numbers range from five to 12, with observed heterozygosities from 0.48 to 0.87. Because two loci are in linkage disequilibrium, these nine loci provide eight independent markers. Three loci exhibit departure from Hardy-Weinberg equilibrium, possibly resulting from null alleles or population admixture. These markers will be useful for assessing population structure and relatedness in S. multiplicata. Based on our success at cross-amplification in the Plains spadefoot toad (Spea bombifrons), these loci also may be useful in this species with additional optimization.

Evolution of pathogen virulence: the role of variation in host phenotype.

Selection on pathogens tends to favour the evolution of growth and reproductive rates and a concomitant level of virulence (damage done to the host) that maximizes pathogen fitness. Yet, because hosts often pose varying selective environments to pathogens, one level of virulence may not be appropriate for all host types. Indeed, if a level of virulence confers high fitness to the pathogen in one host phenotype but low fitness in another host phenotype, alternative virulence strategies may be maintained in the pathogen population. Such strategies can occur either as polymorphism, where different strains of pathogen evolve specialized virulence strategies in different host phenotypes or as polyphenism, where pathogens facultatively express alternative virulence strategies depending on host phenotype. Polymorphism potentially leads to specialist pathogens capable of infecting a limited range of host phenotypes, whereas polyphenism potentially leads to generalist pathogens capable of infecting a wider range of hosts. Evaluating how variation among hosts affects virulence evolution can provide insight into pathogen diversity and is critical in determining how host pathogen interactions affect the phenotypic evolution of both hosts and pathogens.