Systematics and the Origin of Species On Ernst Mayr's 100th Anniversary (2005) / Chapter Skim
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9 Mayr, Dobzhansky, and Bush and the Complexities of Sympatric Speciation in Rhagoletis--JEFFREY L. FEDER, XIANFA XIE, JUAN RULL, SEBASTIAN VELEZ, ANDREW FORBES, BRIAN LEUNG, HATTIE DAMBROSKI, KENNETH E. FILCHAK, AND MARTIN ALUJA
9 Mayr, Dobzhansky, and Bush and the Complexities of Sympatric Speciation in Rhagoletis--JEFFREY L. FEDER, XIANFA XIE, JUAN RULL, SEBASTIAN VELEZ, ANDREW FORBES, BRIAN LEUNG, HATTIE DAMBROSKI, KENNETH E. FILCHAK, AND MARTIN ALUJA
Pages 162-181
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From page 162... ...
Here, we investigate whether these inverted re gions of the genome may have subsequently evolved to become more recalcitrant to introgression relative to collinear regions, consistent with new models for chromosomal speciation. As pre dicted by the models, gene trees for six nuclear loci mapping to chromosomes other than 13 tended to have shallower node *
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From page 163... ...
We discuss the implications of secondary contact and differ ential introgression with respect to sympatric host race formation and speciation in Rhagoletis, reconciling some of the seemingly dichotomous views of Mayr, Dobzhansky, and Bush concerning modes of divergence. E rnst Mayr helped to transform speciation into a holistic science.
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From page 164... ...
pomonella flies may seem paradoxical. The four described and several undescribed sibling species constituting the complex are a model for ecological divergence without geographic isolation by sympatric host plant shifts (Bush, 1966, 1969)
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From page 165... ...
pomonella in the United States originated in a different time and place than the proximate ecological host shifts triggering sympatric divergence. The evidence for past introgression and its contribution to sympatric host shifts could be interpreted as indicating that inversions preferentially flowed from the Mexican Altiplano into the Northern fly population after secondary contact.
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From page 166... ...
Thus, the chromosome model is predicated on Rhagoletis inversions having partially introgressed at a distant time in the past. During subsequent periods of geographic isolation between Mexican and Northern populations, these inverted regions accumulated additional host-related, as well as possibly non-host-related, genetic changes.
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From page 167... ...
We discuss the implications of secondary contact and differential gene flow with respect to sympatric host race formation and speciation in Rhagoletis. MATERIALS AND METHODS Fly Populations Taxa, host plants, collecting sites, and sampling dates for flies are given in Figs.
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From page 168... ...
FIGURE 9.2 MP gene trees for P220 (A)
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From page 169... ...
Gene trees do not include all of the sequenced alleles for each locus, but subsets that encapsulate the general topological structure for trees. Also, networks incorporating recombinant alleles are not shown for P661 and P3060.
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From page 170... ...
electromorpha population. RESULTS AND DISCUSSION Nuclear and mtDNA Gene Trees Of the 19 total nuclear loci analyzed in the study, 4 were determined to be duplicated loci and excluded from further analysis (P341, P2480, P70, and P2919, mapping to chromosomes 1, 3, 3, and 6, respectively)
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From page 171... ...
of recombination events as estimated by the method of Hudson and Kaplan, 1985) , and RND values for loci between major haplotype classes segregating in Mexican and U.S.
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From page 172... ...
. Implications of the Gene Trees: Isolation, Contact, and Differential Gene Flow The tripartite distribution of RNDs for nuclear and mtDNA gene trees is consistent with a hypothesis that Mexican and U.S.
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From page 173... ...
However, we presume that ecological factors related to host phenology affected the clines in the past in a similar manner as they do currently. Loci residing in other regions of the genome not under selection moved readily between Mexican and Northern populations and recombined, accounting for the lack of deep RNDs for chromosome 4 and 5 loci.
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From page 174... ...
flies trace to Mexican origins. Alternative Hypotheses for the Gene Trees The pattern of differentiation seen for nuclear loci could potentially also be explained by incomplete linage sorting of balanced inversion polymorphisms present in the ancestral Mexican/U.S.
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From page 175... ...
Consequently, the observed gene trees are more consistent with the hypothesis of repeated isolation and secondary contact, with inversions on chromosomes 13 becoming increasing more recalcitrant to introgression through time relative to collinear regions of the genome. Our data could also be explained by a series of gene duplication and deletion events within R
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From page 176... ...
flies, the diapause characteristics of the Sierra population match host phenology. Sierra flies eclose significantly earlier than Altiplano flies, resulting in potentially substantial allochronic isolation (J.R., J.L.F., X.X., S
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From page 177... ...
pomonella, the relationship involves a likely sequence of geographic isolation, life-history adaptation, secondary contact, differential introgression, inversion clines, and sympatric host shifts. The evolution of reinforcement can be viewed in an analogous manner, involving nonhost-related traits affecting prezygotic isolation rather than ecological adaptation per se.
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From page 178... ...
rearrangements to introgress implies that these regions of the genome have accumulated additional genetic changes, causing reproductive isolation between Mexican and U.S. flies after their initial establishment in secondary inversion clines in the North.
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From page 179... ...
Two possibilities are male-mediated gene flow and cytonuclear gene interactions affecting host choice. Last, the paleobiology of Mexico and the Southwest must be further investigated to determine whether the distributions of cooccurring fauna and flora, as well as environmental conditions, are consistent with our historical hypothesis for differential gene flow in R
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From page 180... ...
(1969) Sympatric host race formation and speciation in frugivorous flies of the genus Rhagoletis (Diptera, Tephritidae)
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From page 181... ...
(2003b) Evidence for inversion polymorphism related to sympatric host race formation in the apple mag got fly, Rhagoletis pomonella.
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