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Patterns and Mechanisms of Population Diversification in Red-eyed Treefrogs (Agalychnis callidryas)
A central goal in evolutionary biology is to understand the mechanisms underlying the formation of new species (Ott & Endler, 1989; Coyne & Orr, 1998). Population diversification evolves through ecological and evolutionary processes, which could eventually lead to speciation (Coyne & Orr, 1998; Sobel et al., 2009). These processes include both pre- and postzygotic reproductive isolation. Behavioral isolation is a form of prezygotic isolation, which, along with geographic isolation, can serve to reduce gene flow among populations. Postzygotic reproductive isolation evolves via reduced fitness of outcrossed offspring or chromosomal incompatibilities of the parents (Coyne & Orr, 2004; Giovanni, 2008). Examining the relative strength of multiple forms of reproductive isolation provides insight into the mechanisms of divergence. My research provides valuable insight into mechanisms of population divergence and speciation by focusing on the consequences that phenotypic and genetic diversification can have on both prezygotic and postzygotic reproductive isolation. The first chapter of my thesis examines pre-mating behavioral isolation by testing female mate preference between two divergent populations of red-eyed treefrog. We found that females in each population preferred local mates. Further, females largely chose males in the absence of any male advertisement call, underscoring the importance of non-acoustic cues in mate choice, a known sexual signal for most anurans. This research provides evidence that population divergence likely has consequences for mate recognition and choice. My second chapter examined variation in sperm response to an exogenous hormone among four differentiated populations in Costa Rica. Differences in sperm production in response to the exogenous hormone LHRH (Luteinizing hormone releasing hormone) could indicate that populations have differentiated at a physiological level, which could have consequences for postmating reproductive isolation and eventual lineage divergence. This chapter had two goals. The first was to examine variation among populations in sperm production. We hypothesized that differences in receptivity to hormones or the endocrine system have also evolved among these differentiated populations. To our knowledge, no study has examined sperm response to exogenous hormones across highly differentiated populations within a species. We found that although sperm count did not vary among populations, at least one population showed decreased sperm viability in response to the hormone. From an evolutionary perspective it appears that one population has differentiated in receptiveness to hormone injections. The second goal of this chapter was to develop an Assisted Reproductive Technologies (ART) protocol that could later be used for laboratory breeding, conservation techniques and further studies on reproductive isolation. From a conservation perspective the hormone was effective in producing viable sperm. Thus, we propose that the hormone LHRH is sufficient for conservation protocols for most populations of red-eyed treefrogs. However, for developing an ART protocol for species with highly differentiated populations, we recommend that starting with a baseline dosage efficient at inducing spermiation is adequate but specialized protocols for spermiation, captive breeding or studies on postzygotic reproductive isolation may be needed for species with highly differentiated populations across their range. Combined, the chapters underscore the importance of examining divergence among populations and how population differentiation can lead pre and postmating reproductive isolation and eventual lineage divergence. In addition, the differences among populations can affect conservation and laboratory breeding protocols.