Cruzamentos experimentais revelam estágios iniciais de especiação em Akodon cursor (Rodentia, Cricetidae)
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Data
2025-08-14
Autores
Haase, Amanda Freitas
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Universidade Federal do Espírito Santo
Resumo
Akodon cursor Winge, 1887 is a remarkable species due to its wide geographic distribution across the eastern Atlantic Forest, spanning an extensive latitudinal gradient of over 2,500 km— from Paraíba to northern Paraná—covering the Brazilian states of Pernambuco, Bahia, Minas Gerais, Espírito Santo, Rio de Janeiro, and São Paulo. The species exhibits high karyotypic diversity, with 31 recognized karyotypes varying in diploid number (2n = 14, 15, and 16), unevenly distributed across populations. It also presents a well-supported phylogeographic structure comprising three major clades (North, Central, and South), each containing individuals with all three diploid numbers. These features have raised the hypothesis that A. cursor may comprise more than one taxon, potentially differentiated by karyotype, geography, or both. This study employed intra- and interpopulation experimental crossbreeding to assess reproductive barriers among populations representative of the species’ geographic, phylogenetic, and karyotypic variability: Pernambuco (PE, North clade, 2n = 16), Bahia (BA, North clade, 2n = 14, 15, and 16), and Espírito Santo (ES, South clade, 2n = 14). A total of 55 matrices (31 males and 24 females) were collected from the wild between 2013 and 2016, generating 495 live offspring through 334 controlled crosses. Of these, 66 intrapopulation crosses established baseline reproductive parameters for the species: Reproductive Success (RS, i.e., proportion of crosses resulting in offspring), Litter Size (LS), and Gestation Length in days (GL). RS was analyzed using Bayesian Generalized Linear Models (GLM), and LS was assessed via Kruskal–Wallis tests followed by Conover–Iman post hoc comparisons. Interpopulation crosses were used to investigate the presence of reproductive barriers. Intrapopulation crosses showed, on average, RS = 56.06%, LS = 3.7 ± 1.04, and GL = 24.6 ± 1.69. Among interpopulation crosses, reduced RS was observed in ♂PE16×♀ES14 (RS = 26.67%) and ♂PE16×♀BA14–16 (RS = 0), suggesting incomplete reproductive barriers. In contrast, BA14×ES14 crosses (RS = 33%) showed no barrier. The resulting F1 hybrids with 2n = 15 (F115) were sterile males and fertile females in the interpopulation PE16×ES14 cross, whereas in the intrapopulation BA16×BA14 cross, F115 were fertile (RS = 14.3%) despite reduced RS. Based on RS data, our results do not support the existence of complete reproductive isolation among these populations. The production of sterile F1 males and fertile females in ES14×PE16 crosses aligns with Haldane’s Rule, which describes patterns of postzygotic isolation. Additionally, reduced RS values are consistent with asymmetric reinforcement, supported by the lower fitness of F1 hybrids between these populations. Notably, RS reduction patterns do not align with the species’ phylogeographic structure, as lower RS was observed in crosses between poix pulations within the same clade (BA and PE). Reproductive performance of 2n = 15 individuals ranged from complete sterility to partial fertility, depending on the parental combination. We propose, therefore, a complex model of incipient speciation in Akodon cursor, shaped by geographic and karyotypic factors and involving multiple degrees of reproductive isolation.
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Palavras-chave
Especiação cromossômica , Isolamento reprodutivo assimétrico , Regra de Haldane