Mate preference: a possible causal mechanism for a moving hybrid zone (original) (raw)
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Extrapair paternity and mate choice in a chickadee hybrid zone
Behavioral Ecology, 2005
The dynamics of hybrid zones are likely to be influenced greatly by patterns of mate choice, including ''cryptic'' choice mediated through extrapair copulations. To understand changes in hybrid zones over time and space, a detailed examination of mating patterns and correlates is needed. We studied the role of extrapair fertilizations (EPFs) in the breeding biology of hybridizing black-capped and Carolina chickadees in southeastern Pennsylvania over 4 years, using microsatellite DNA markers. We detected extrapair offspring (EPO) in 56% of 90 broods examined; these accounted for at least 26% of 477 offspring. Chickadees do not appear to use EPFs to reduce costs of heterospecific pairing: EPFs were no more likely to occur in genetically dissimilar (heterospecific) social pairs than in pairs where social mates were genetically similar. However, females paired with black-capped-like males were more likely to have EPO. Females that acquired EPFs did not obtain these from males genetically similar to themselves; instead, all females, regardless of their genotype or that of their social mate, tended to prefer Carolina-like males as extrapair partners. There was no relationship between the presence of EPO and hatching or fledging success. High rates of extrapair paternity and apparent female preference for Carolina-like males suggest that mate choice is an important influence in ongoing northward movement of this hybrid zone.
The Auk, 2005
AяѠѡџюѐѡ.-Black-capped Chickadees (Poecile atricapillus) and Carolina Chickadees (P. carolinensis) hybridize in an east-west band from New Jersey to Kansas. Within the past century, the Ohio portion of this hybrid zone and the Carolina Chickadee range to the south have been moving northward, whereas the Black-capped Chickadee range has retracted. In Ohio, we characterized the genetic composition of the hybrid zone using fi ve diagnostic molecular loci. Although there was no evidence of assortative mating in the center of the hybrid zone, we found a relative paucity of genetically intermediate breeding females as compared with breeding males. That suggests viability selection against female hybrids, in line with Haldane's rule. On the basis of reproductive variables (number of nestlings, reproductive success), we found a decrease in productivity of breeding pairs in the hybrid zone that is signifi cantly and positively related to their probability of producing homozygous off spring at each autosomal or sex-linked locus. We also found that the decrease in productivity was signifi cantly and positively related to the genetic composition of the male of the pair (i.e. pure male chickadees more productive). These data strongly suggest that hybrids are at a selective disadvantage. Because the zone of reduced reproductive success was considerably narrower than the zone of introgression, our results demonstrate that genetic introgression is occurring in the face of substantial selection against hybrids.
The Auk, 2007
Analysis of the structure and stability of a hybrid zone can serve as a starting point for examining mechanisms that infl uence spatial and evolutionary relationships between species. Recent studies of the hybrid zone between Blackcapped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolinensis) have suggested that genetic introgression is limited to a narrow zone, while also reinforcing the conclusion that the line of contact between these parapatrically distributed species is now shift ing northward. We investigated the structure, position, and recent movement of the chickadee hybrid zone in southeastern Pennsylvania. Using selectively neutral microsatellite DNA markers, along with mitochondrial DNA haplotypes, we documented large diff erences in genetic composition among three populations ≤65 km apart where we sampled large numbers of breeding residents during 1998-2003. Genetic results indicate that the three sites support a population of Carolina Chickadees (Great Marsh), a population in which most individuals exhibit evidence of hybridization (Nolde Forest), and a population comprising mostly Black-capped Chickadees but with evidence of hybridization now taking place (Hawk Mountain). The patt erns within the Nolde Forest population suggest that selection against hybrids may not be as strong as has been concluded from studies in other parts of the chickadee hybrid zone. Comparison of mitochondrial and nuclear genotypes between samples collected ∼15 years apart suggest that the northern edge of the hybrid zone shift ed by ∼20 km over this interval, with hybridization now occurring as far north as the Kitt atinny Ridge and beyond, where only Blackcapped Chickadee genotypes were previously detectable. Our data and historical accounts suggest that the hybrid zone, now ∼50 km wide, may have become wider while also shift ing northward. These results support the hypothesis that Carolina Chickadees enjoy a selective advantage during hybridization with Black-capped Chickadees, but both the proximate mechanisms and ultimate causes remain to be investigated.
BOUNDED HYBRID SUPERIORITY IN AN AVIAN HYBRID ZONE: EFFECTS OF MATE, DIET, AND HABITAT CHOICE
Evolution, 2000
There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.
Patterns of introgression vary within an avian hybrid zone
BMC Ecology and Evolution, 2021
BackgroundExploring hybrid zone dynamics at different spatial scales allows for better understanding of local factors that influence hybrid zone structure. In this study, we tested hypotheses about drivers of introgression at two spatial scales within the Saltmarsh Sparrow (Ammospiza caudacuta) and Nelson’s Sparrow (A. nelsoni) hybrid zone. Specifically, we evaluated the influence of neutral demographic processes (relative species abundance), natural selection (exogenous environmental factors and genetic incompatibilities), and sexual selection (assortative mating) in this mosaic hybrid zone. By intensively sampling adults (n = 218) and chicks (n = 326) at two geographically proximate locations in the center of the hybrid zone, we determined patterns of introgression on a fine scale across sites of differing habitat. We made broadscale comparisons of patterns from the center with those of prior studies in the southern edge of the hybrid zone.ResultsA panel of fixed SNPs (135) identi...
The Auk, 2000
We studied hybridization and introgression between Black-capped (Poecile atricapillus) and Carolina (P. carolinensis) chickadees along two transects in the Appalachians using four genetic markers and multivariate analysis of morphology. Genetic data revealed that at least 58% of the birds in the center of each transect were of mixed ancestry and that recombinant genotypes predominated among hybrids, demonstrating that hybridization is frequent and that many hybrids are fertile. Genetic clines generally were steep and coincident in position, but introgression was evident well beyond the range interface. Introgression was higher at the one autosomal locus surveyed than in mitochondrial DNA or in two sexlinked markers, suggesting that the hybrid zone is a conduit for gene flow between the two forms at some loci. On a broad scale, morphometric variation was concordant with genetic variation. Clines in morphological variation based on principal components (PC) scores were steep and coincident with genetic clines. Also, a strong correlation within a population between PC scores and an individual's genetic makeup suggested that a large amount of morphological variation was genetically determined. However, morphological analysis indicated that hybrids were uncommon on one transect, whereas genetic data clearly showed that they were common on both. In addition, patterns of morphological variation were equivocal regarding introgression across the hybrid zone. Thus, genetic data provided a complementary and more detailed assessment of hybridization, largely due to the discrete nature of genetic variation. Genetic markers are useful in understanding hybridization and introgression, but diagnostic markers may underestimate average gene flow if selection against hybrids maintains steep clines at diagnostic loci. To gain a clearer picture of the genome-wide effects of hybridization, a much larger number of loci must be assayed, including non-diagnostic ones.
Habitat preferences and positive assortative mating in an avian hybrid zone
Journal of Avian Biology, 2010
The relatively narrow hybrid zone between hooded and carrion crow is characterised by a high proportion of breeding hybrids, no known intrinsic postzygotic costs, and most likely, high gene flow between populations. It is therefore of interest to identify factors preventing a breakdown of the zone. In this study, we investigate the possibility that habitat choice could create assortative mating in the GermanÁDanish part of the zone. In a GIS analysis, we used two approaches to investigate the role of habitat selection in the maintenance of this zone. We tested if there are 1) any correlations between habitat and the shape of the hybrid zone, and 2) any differences in habitat preferences between territorial individuals of the crow phenotypes. The most extensive habitat, non-irrigated arable land (Habitat 1), which covers almost 50% of the area in the zone was preferred by all three phenotypes (carrion, hooded and hybrid crows), but showed no change in frequency across the zone. Pastures (Habitat 2) and agricultural areas mixed with natural vegetation (Habitat 4) correlated with the shape of the zone, but only H4 differed in preference between the crow phenotypes. H4 was preferred by hybrids and hooded crows, but not by carrion crows. This habitat is present only in 17% of the hooded crow territories, and thus, it is not likely to have a strong influence on the maintenance of the hybrid zone. There was evidence of assortative mating in pure phenotypes, whereas hybrids showed no consistent mating pattern. Because our GIS analyses suggest that species specific habitat preferences are not involved in shaping the assortativeness, we discuss other possible mechanisms.
2001
In western Panama, an unusual hybrid zone exists between white-collared manakins, Manacus candei, and golden-collared manakins, M. vitellinus. Unidirectional introgression of plumage traits from vitellinus into candei has created a region in which all definitively plumaged males have a collar that is lemon-colored. These males are nearly indistinguishable from white-collared candei genetically and morphometrically, but strongly resemble golden-collared vitellinus due to the introgression of secondary sexual plumage traits, particularly the lemon-colored collar. The introgression could be explained by sexual selection for golden-collared traits or by a series of mechanisms that do not invoke sexual selection (e.g., neutral diffusion, dominant allele). Sexual selection on male-male interactions implies behavioral differences among the plumage forms-specifically that golden-and lemon-collared males should be more aggressive than white-collared males. In contrast, the nonsexual hypotheses predict behavioral similarity between lemon-and white-collared males, based on their nearly identical genetics. We tested the sexual selection hypothesis experimentally, by presenting males with taxidermic mounts of the three forms. As response variables, we monitored vocalizations and attacks on the mounts by replicate subject males. Both golden-collared and lemon-collared males were more likely to attack than were white-collared males, as predicted under sexual selection but not by the nonsexual hypotheses. Lemon-collared males were more vocally reactive than either parental form, contrary to the prediction of the nonsexual hypotheses. Our study demonstrates that sexual selection on male-male interactions may play an important role in the dynamics of character evolution and hybrid zones.
Maintenance of a hybrid zone: The role of female mate choice
The Auk, 2011
Hybrid zones between different plumage morphs are common in birds. These zones can be maintained by (1) divergent selection pressures on either side of the zone or (2) some restriction to mating between the forms that limits gene flow from one side to the other. In eastern Australia, there is a distinct hybrid zone between two plumage forms of the Australian Magpie (Cracticus tibicen), with black-backed birds in the north, white-backed birds in the south, and both forms plus intermediates in a zone ~100 km wide. On the basis of social groups, there is no evidence of assortative mating in the hybrid zone. However, extrapair fertilizations (EPF) occur in other Australian Magpie populations and, thus, may also occur in the hybrid zone. We examined evidence of EPFs in the hybrid zone to test for either (1) positive assortative mating or (2) preference for the brighter plumage form. Although there were significant levels of EPFs of ~30%, there was no evidence for positive assortative mating or preference for the brighter white-backed males. Other explanations for the current distribution of the hybrid zone and its maintenance will need to be investigated.