Contact zone dynamics during early stages of speciation in a chorus frog (Pseudacris crucifer)

Abstract

Characterizing the genetic and behavioural consequences of contact between previously geographically isolated lineages provides insights into the mechanisms underlying diversification and ultimately speciation. The spring peeper (Pseudacris crucifer) is a widespread Nearctic chorus frog with six divergent mitochondrial DNA (mtDNA) lineages, many of which came into secondary contact during the Holocene. We examined genetics, morphology, advertisement calls and female preference for two lineages that began diverging in allopatry in the Pliocene and now overlap in southwestern Ontario, Canada. We found non-coincident clines in mtDNA and nuclear DNA, mirroring directionality of premating isolation barriers. We also found divergence in a range of traits between these two lineages, displacement in male call attributes and female preference for calls of their natal lineage in sympatry. Hybrids were morphologically distinct from both parental lineages, but hybrid male calls were acoustically intermediate. Female hybrids showed asymmetrical preference for Eastern male calls. These results considered together provide evidence of either unidirectional hybridization or selection against hybrids, potentially implying reproductive character displacement. Our work demonstrates the utility of integrated, multi-character approaches to understanding the processes of divergence and the nature of speciation.

Can alternative mating tactics facilitate introgression across a hybrid zone by circumventing female choice?

“…Similar to Stewart et al . (), our microsatellite‐based STRUCTURE analysis revealed no pure I individuals (apparent asymmetry to hybridization within the contact zone), but did identify six pure E individuals, 10 putative F1 hybrids with Eastern haplotypes (HE), 25 putative F1 hybrids with Interior haplotypes (HI) and six individuals with Interior haplotypes but with pure Eastern nuclear alleles, presumably representing HI backcrosses to Eastern individuals (BE). We were unable to sequence the mtDNA for two individuals from different dyad pairs, and genotype one individual for microsatellites (a satellite male to an individual whose mtDNA could not be sequenced), reducing our sample size to 23 dyad pairs, 47 individuals within the contact zone.…”

“…Populations from which caller–satellite dyads were sampled are represented by a black star. The area outlined in a grey‐dashed line signifies the contact zone, where both Interior and Eastern mt DNA haplotypes are found within the same breeding populations (Austin et al ., , ; Stewart et al ., ). Inset: distribution of six mt DNA lineages (separated by dashed lines) across North America, with the Southwestern Ontario contact zone bordered by a black square.…”

“…Each individual was sequenced for a 692‐base pair segment of mitochondrial cytochrome b (cyt b ) and genotyped for 11 previously published nuclear DNA microsatellite loci (Degner et al ., ; Stewart et al ., ) according to protocols outlined in Stewart et al . ().…”

“…(). To identify hybrids, we used genotypic data from 428 reproductively active spring peeper adults from 14 Southwestern Ontario populations, with dyad individuals included (see Stewart et al ., ; Fig. ).…”

“…() method, to identify the most likely value of K clusters. Based on STRUCTURE (K = 2) and HYBRIDLAB version 1.0 (Nielson et al ., ) simulations to evaluate our ability to diagnose distinct genotypic classes within this contact zone, an individual's genetic background was designated as: hybrid (H) if 0.15 < q > 0.85, pure Eastern (E) if q < 0.15 and pure Interior (I) if q > 0.85 (see Stewart et al ., for justification).…”