Mating patterns and post-mating isolation in three cryptic species of the Engystomops petersi species complex

Abstract

Determining the extent of reproductive isolation in cryptic species with dynamic geographic ranges can yield important insights into the processes that generate and maintain genetic divergence in the absence of severe geographic barriers. We studied mating patterns, propensity to hybridize in nature and subsequent fertilization rates, as well as survival and development of hybrid F1 offspring for three nominal species of the Engystomops petersi species complex in Yasuní National Park, Ecuador. We found at least two species in four out of six locations sampled, and 14.3% of the wild pairs genotyped were mixed-species (heterospecific) crosses. We also found reduced fertilization rates in hybrid crosses between E. petersi females and E. “magnus” males, and between E. “magnus” females and E. “selva” males but not in the reciprocal crosses, suggesting asymmetric reproductive isolation for these species. Larval development times decreased in F1 hybrid crosses compared to same species (conspecific) crosses, but we did not find significant reduction in larval survival or early metamorph survival. Our results show evidence of post-mating isolation for at least two hybrid crosses of the cryptic species we studied. The general decrease in fertilization rates in heterospecific crosses suggests that sexual selection and reinforcement might have not only contributed to the pattern of call variation and behavioral isolation we see between species today, but they may also contribute to further signal divergence and behavioral evolution, especially in locations where hybridization is common and fertilization success is diminished.

Peripheral Auditory System Divergence Does Not Explain Species Differences in Call Preference

“…To estimate the response to natural calls we used a sample of calls of the three species recorded in La Selva Lodge and Yasuní National Park. Recordings included original calls from Boul et al [2007] and frogs recorded in a captive colony at Yasuní Research Station later identified genetically (as reported in Trillo et al [2017]). Calls were recorded using a Sennheiser SE66 microphone [Boul et al, 2007] or an Olympus LS-10 digital recorder.…”

“…Second, we estimated the detectability of signals without modeling cross-frequency interference or background noise. These cryptic species overlap in ranges, mating at times in the same pond on the same night and occasionally hybridizing [Trillo et al, 2017], so we know some individuals from each species are subject to similar background noise in their current distribution. Neural responses to complex acoustic signals in complex acoustic backgrounds as well as transmission through the environment will modify the actual conspicuousness of calls in natural settings.…”

“…Moreover, only E. 'magnus' females significantly prefer complex calls over simple calls (86% choosing complex calls) [Boul et al, 2007], although results from E. petersi females are equivocal [Guerra and Ron, 2008]. All three species overlap in distribution and can mate simultaneously at the same mating ponds, which leads to occasional amplexus between species [Trillo et al, 2017]. Thus, these cryptic species differ in various male call features and female preferences that nominate a possible role in speciation for sexual selection based on acoustic communication.…”

“…We focus on three proposed species of the E. petersi complex that diverged an estimated 6-16 million years ago [Funk et al, 2012] in which both male and female reproductive behaviors have been characterized. We refer to the species by the preliminary names referenced in Trillo et al [2017], as formal species descriptions and names have not yet been published. Figure 1 shows the naming scheme and inferred relationships among these species from Funk et al [2012].…”

Follow-up ecological studies for cryptic species discoveries: Decrypting the leopard frogs of the eastern U.S.

“…Confirmation of cryptic diversity in cavefish [ 17 ] was followed by conservation status assessments that showed many lineages to be of conservation concern [ 18 ]. And Trillo et al [ 19 ] followed the elucidation of cryptic diversity in Amazonian frogs [ 20 ] with a study of behavioral isolating mechanisms.…”