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7 results for “towhee”
Hybrid cline or hybrid lineage: A genomic reevaluation of Sibley's classic species conundrum in Pipilo towhees
<p>Hybrid zones are often described as clines of genetic and phenotypic traits moving across species barriers through introgression. Yet, hybrid zones can also be spatially complex and shift over time, and dispersal and vicariance can isolate portions of a cline, potentially leading to hybrid lineage formation. We reassessed Sibley's (1950) gradient between Collared Towhee (Pipilo ocai) and Spotted Towhee (P. maculatus) in Central Mexico to test whether it conformed to a typical tension-zone cline model. By comparing historical and modern data, we found that cline centers for genetic and phenotypic traits have not shifted over the course of 70 years. This equilibrium suggests that secondary contact between these species, which originally diverged over 2 million years ago, likely dates to the Pleistocene. Given the amount of mtDNA divergence, parental ends of the cline have very low autosomal nuclear differentiation (FST = 0.12). Dramatic and coincident cline shifts in mtDNA and throat color suggest the possibility of sexual selection as a factor in differential introgression, while a contrasting cline shift in green back color hints at a role for natural selection. Supporting the idea of a continuum between hybrid clines and hybrid lineage formation, the towhee gradient can be analyzed as one population under isolation-by-distance, as a two-population cline, and as three lineages experiencing divergence with gene flow. In the middle of the gradient, a hybrid lineage has become partly isolated, likely due both to forested habitat shrinking and fragmenting as it moved upslope after the last glacial maximum and a stark environmental transition. The towhee system offers a window into the potential outcomes of hybridization across a dynamic landscape including the creation of novel genomic and phenotypic combinations and incipient hybrid lineages.</p>
Hybrid cline or hybrid lineage: A genomic reevaluation of Sibley’s classic species conundrum in Pipilo towhees
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Data from: Heterogeneity and concordance in locus-specific differentiation and introgression between species of towhees
The maintenance or breakdown of reproductive isolation are observable outcomes of secondary contact between species. In cases where hybrids beyond the F1 are formed, the representation of each species' ancestry can vary dramatically among genomic regions. This genomic heterogeneity in ancestry and introgression can offer insight into evolutionary processes, particularly if introgression is compared in multiple hybrid zones. Similarly, considerable heterogeneity exists across the genome in the extent to which populations and species have diverged, reflecting the combined effects of different evolutionary processes on genetic variation. We studied hybridization across two hybrid zones of two phenotypically well-differentiated bird species in Mexico (Pipilo maculatus and P. ocai), to investigate genomic heterogeneity in differentiation and introgression. Using genotyping by sequencing (GBS) and hierarchical Bayesian models, we genotyped 460 birds at over 41,000 single nucleotide polymorphism (SNP) loci. We identified loci exhibiting extreme introgression relative to the genome-wide expectation using a Bayesian genomic cline model. We also estimated locus-specific FST and identified loci with exceptionally high genetic divergence between the parental species. We found some concordance of locus-specific introgression in the two independent hybrid zones (6-20% of extreme loci shared across zones), reflecting areas of the genome that experience similar gene flow when the species interact. Additionally, heterogeneity in introgression and divergence across the genome revealed another subset of loci under the influence of locally specific factors. These results are consistent with a history in which reproductive isolation has been influenced by a common set of loci in both hybrid zones, but where local environmental and stochastic factors also lead to genomic differentiation.
Data for: Lower survival of hybrid grosbeaks, but not towhees, suggests a molt divide disfavors hybrids
<p>Although avian hybrid zones in the Great Plains have been studied for almost 70 years, we know surprisingly little about the fitness costs for hybrids that keep these zones narrow. We compare age ratios in grosbeaks (<em>Pheucticus ludovicianus</em> and <em>P. melanocephalus</em>) and towhees (<em>Pipilo erythropthalums</em> and <em>P. maculatus</em>), two species pairs that differ in their life-histories and molt schedules, to evaluate survival between hybrids and parentals. We then contrast molt and migratory divides as possible sources of selection against hybrids. Hybrid grosbeaks had 27-33% lower survival relative to their parentals, whereas hybrid towhees had survival rates similar to parentals. Age ratio data for hybrid grosbeaks suggest high mortality in older birds, as expected if selection operates after the first year of life. This pattern is consistent with parental species of grosbeaks having contrasting molt schedules relative to migration, suggesting high mortality costs to hybrids driven by molt biology, which are expressed later in life. Contrasts in molt schedules are absent in towhees. While migratory divides may exist for towhees and grosbeaks, the low adult survival of hybrid grosbeaks suggests that molt may be an important and underappreciated source of selection maintaining this and other narrow avian hybrid zones.</p>
Data from: Heterogeneity and concordance in locus-specific differentiation and introgression between species of towhees
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Data for: Lower survival of hybrid grosbeaks, but not towhees, suggests a molt divide disfavors hybrids
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Data from: Spatial patterns of extra-pair paternity for spotted towhees (Pipilo maculatus) in urban parks
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