The population genetics of speciation by cascade reinforcement

Abstract

Species interactions drive diverse evolutionary outcomes. Speciation by cascade reinforcement represents one example of how species interactions can contribute to the proliferation of species. This process occurs when the divergence of mating traits in response to selection against interspecific hybridization incidentally leads to reproductive isolation among populations of the same species. Here, we investigated the population genetic outcomes of cascade reinforcement in North American chorus frogs (Hylidae: Pseudacris). Specifically, we estimated the frequency of hybridization among three taxa, assessed genetic structure within the focal species, P. feriarum, and ascertained the directionality of gene flow within P. feriarum across replicated contact zones via coalescent modeling. Through field observations and preliminary experimental crosses, we assessed whether hybridization is possible under natural and laboratory conditions. We found that hybridization occurs among P. feriarum and two conspecifics at a low rate in multiple contact zones, and that gene flow within the former species is unidirectional from allopatry into sympatry with these other species in three of four contact zones studied. We found evidence of substantial genetic structuring within P. feriarum including a divergent western allopatric cluster, a behaviorally-distinct sympatric South Carolina cluster, and several genetically-overlapping clusters from the remainder of the distribution. Furthermore, we found sub-structuring between reinforced and nonreinforced populations in the two most intensely-sampled contact zones. Our literature review indicated that P. feriarum hybridizes with at least five heterospecifics at the periphery of its range providing a mechanism for further intraspecific diversification. This work strengthens the evidence for cascade reinforcement in this clade, revealing the geographic and genetic landscape upon which this process can contribute to the proliferation of species.

Keywords: cascade reinforcement; character displacement; hybridization; population genetics; speciation.

FIGURE 1

Representative schematic of the study system and population genetic analyses conducted. The ranges of Pseudacris feriarum and eight congeneric taxa with which the focal species experiences some degree of range overlap are shown in transparent overlays. The five heavily‐sampled contact zones between P. feriarum and at least one congener in this study are represented by pairs of P. feriarum populations that are sympatric (triangle) and allopatric (square) with respect to either P. nigrita or both P. nigrita and P. brimleyi. The two sympatric populations included in our analyses of interspecific hybridization are symbolized by white triangles, and the remaining three sympatric populations are symbolized by black triangles. Bidirectional arrows represent our estimation of the directionality of gene flow between allopatric and sympatric pairs of P. feriarum populations.

FIGURE 2

Hybrid indexes (h) for individuals collected at two contact zones between species. In all cases, values near 1 or 0 indicate “pure” genotypes of one of the species involved in the analysis. Intermediate values indicate admixed individuals (filled circles) with different genetic proportions of each species. (a) P. feriarum (h ~ 1) vs. P. nigrita (h ~ 0) in the Apalachicola River river drainage of Florida (FL). (b) P. feriarum (h ~ 1) vs. P. nigrita (h ~ 0) in Edisto‐Santee river drainage of South Carolina (SC). (c) P. brimleyi (h ~ 1) vs. P. feriarum (h ~ 0) in the Edisto‐Santee river drainage of South Carolina (SC). In panel (c), asterisks mark two natural hybrids captured in the field; the remaining seven hybrids were lab‐generated. Error bars represent confidence intervals for each hybrid index estimate (Tables S24–S26).

FIGURE 3

Geographic locations and admixture coefficients within the Florida (FL) contact zone. Analyses were conducted in fastSTRUCTURE with P. feriarum and P. nigrita for the most likely cluster configurations (bolded, K = 3 and K = 4 were best‐supported; Figure S2; Table S27). Gray‐shaded area of the map represents the range of P. nigrita. For K = 3 (a) and K = 4 (b), dark gray indicates sympatric and allopatric P. nigrita, green indicates sympatric and allopatric P. feriarum, and purple indicates western allopatric P. feriarum. Three colors are shown at K = 4 (b) since the fourth genetic cluster had very low admixture coefficients that could not be visualized (Table S13). At K = 5 (c), dark green indicates an additional allopatric P. feriarum cluster and lime green indicates a sympatric P. feriarum cluster. Four colors are shown at K = 4 (c) since the fifth genetic cluster had very low admixture coefficients that could not be visualized (Table S14).

FIGURE 4

Geographic locations and admixture coefficients within the South Carolina (SC) contact zone. Analyses were conducted in fastSTRUCTURE with P. feriarum, P. nigrita, and P. brimleyi for the most likely cluster configuration (bolded, K = 4 was best‐supported; Figure S3; Table S27). Gray‐shaded area of the map represents the range of P. nigrita. For K = 3 to K = 5 (a–c), dark gray indicates sympatric and allopatric P. nigrita, and light blue gray indicates sympatric and allopatric P. brimleyi. At K = 3 (a), orange represents all P. feriarum; at higher levels of K, however, coastal sympatric populations from the Charleston, South Carolina (SC) area (orange) cluster separately from the other mainly allopatric conspecific populations (medium blue).

FIGURE 5

Geographic locations and admixture coefficients for all sequenced P. feriarum. Analyses were conducted in fastSTRUCTURE for the most likely cluster configurations (bolded, K = 3 and K = 5 were best‐supported; Figure S4). Gray‐shaded area of the map represents the range of P. nigrita. (a) K = 3, (b) K = 4, and (c) K = 5. For K = 5, purple indicates western allopatric populations, green indicates sympatric and allopatric populations west and south of the Appalachian mountains, orange indicates the sympatric South Carolina populations, yellow indicates an inland allopatric cluster west of the Appalachian mountains, and blue corresponds to the remaining mostly allopatric populations.

FIGURE 6

Population clusters within P. feriarum (K = 7) as defined by a discriminant analysis of principal components (DAPC). (a) Map showing the geographic position of the sampled P. feriarum individuals with colors indicating the assigned DAPC cluster. Gray‐shaded areas of the map indicate topography (darker areas correspond to higher elevation). (b) Scatter plot showing the population clusters as resulting from the first two discriminant functions (eigenvalues in the inset bar plot). DAPC clusters each include individuals from areas of sympatry with P. nigrita and/or P. brimleyi, as well as allopatric areas, except for the South Carolina (orange) and Western (purple) clusters, which include sympatric and allopatric individuals, respectively.