Non-recombining sex chromosomes are expected to undergo evolutionary decay, ending up genetically degenerated, as has happened in birds and mammals. Why are then sex chromosomes so often homomorphic in cold-blooded vertebrates? One possible explanation is a high rate of turnover events, replacing master sex-determining genes by new ones on other chromosomes. An alternative is that X-Y similarity is maintained by occasional recombination events, occurring in sex-reversed XY females. Based on mitochondrial and nuclear gene sequences, we estimated the divergence times between European tree frogs (Hyla arborea, H. intermedia, and H. molleri) to the upper Miocene, about 5.4–7.1 million years ago. Sibship analyses of microsatellite polymorphisms revealed that all three species have the same pair of sex chromosomes, with complete absence of X-Y recombination in males. Despite this, sequences of sex-linked loci show no divergence between the X and Y chromosomes. In the phylogeny, the X and Y alleles cluster according to species, not in groups of gametologs. We conclude that sex-chromosome homomorphy in these tree frogs does not result from a recent turnover but is maintained over evolutionary timescales by occasional X-Y recombination. Seemingly young sex chromosomes may thus carry old-established sex-determining genes, a result at odds with the view that sex chromosomes necessarily decay until they are replaced. This raises intriguing perspectives regarding the evolutionary dynamics of sexually antagonistic genes and the mechanisms that control X-Y recombination.

Phylogeographic studies are often focused on temperate European species with relict footholds in the Mediterranean region. Past climatic oscillations usually induced range contractions and expansions from refugial areas located in southern Europe, and spatial distribution of genetic diversity show that northward expansions were usually pioneer-like. Actually, few studies have focused on circum-Mediterranean species, which probably were not influenced in the same way by climatic oscillations. We present the phylogeography of the bark beetle Tomicus destruens, which is restricted to the whole Mediterranean basin and the Atlantic coasts of North Africa and Portugal. We systematically sequenced 617 bp of the mitochondrial genes COI and COII for 42 populations (N = 219). Analysis revealed 53 haplotypes geographically structured in two clades, namely eastern and western clades, that diverged during the Pleistocene. A contact zone was identified along the Adriatic coast of Italy. Interestingly, we found contrasting levels of genetic structure within each clade. The eastern group was characterized by a significant phylogeographic pattern and low levels of gene flow, whereas the western group barely showed a spatial structure in haplotype distribution. Moreover, the main pine hosts were different between groups, with the Aleppo-brutia complex in the east and the maritime pine in the west. Potential roles of host species, climatic parameters and geographical barriers are discussed and the phylogeographic patterns are compared to classical models of postglacial recolonization in Europe.

Contrasting with birds and mammals, poikilothermic vertebrates often have homomorphic sex chromosomes, possibly resulting from high rates of sex-chromosome turnovers and/or occasional X-Y recombination. Strong support for the latter mechanism was provided by four species of European tree frogs, which inherited from a common ancestor (∼ 5 Ma) the same pair of homomorphic sex chromosomes (linkage group 1, LG1), harboring the candidate sex-determining gene Dmrt1. Here, we test sex linkage of LG1 across six additional species of the Eurasian Hyla radiation with divergence times ranging from 6 to 40 Ma. LG1 turns out to be sex linked in six of nine resolved cases. Mapping the patterns of sex linkage to the Hyla phylogeny reveals several transitions in sex-determination systems within the last 10 My, including one switch in heterogamety. Phylogenetic trees of DNA sequences along LG1 are consistent with occasional X-Y recombination in all species where LG1 is sex linked. These patterns argue against one of the main potential causes for turnovers, namely the accumulation of deleterious mutations on nonrecombining chromosomes. Sibship analyses show that LG1 recombination is strongly reduced in males from most species investigated, including some in which it is autosomal. Intrinsically low male recombination might facilitate the evolution of male heterogamety, and the presence of important genes from the sex-determination cascade might predispose LG1 to become a sex chromosome.

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Chromosomal rearrangements are proposed to promote genetic differentiation between chromosomally differentiated taxa and therefore promote speciation. Due to their remarkable karyotypic polymorphism, the shrews of the Sorex araneus group were used to investigate the impact of chromosomal rearrangements on gene flow. Five intraspecific chromosomal hybrid zones characterized by different levels of karyotypic complexity were studied using 16 microsatellites markers. We observed low levels of genetic differentiation even in the hybrid zones with the highest karyotypic complexity. No evidence of restricted gene flow between differently rearranged chromosomes was observed. Contrary to what was observed at the interspecific level, the effect of chromosomal rearrangements on gene flow was undetectable within the S. araneus species.

1 The Mediterranean pine shoot beetle Tomicus destruens has long been indistinguishable from its congeneric Tomicus piniperda . Both species attack pines, and can be found in sympatry. The geographical distribution of T. destruens is still unclear in most of the Mediterranean Basin. 2 We aimed to describe the geographical distribution and zones of sympatry of both species in the Iberian Peninsula and France, and to study the molecular phylogeographical pattern of T. destruens . 3 Tomicus spp. adults were sampled in Portugal, Spain and France, and a portion of the mitochondrial genes COI and COII was sequenced for 84 individuals. Sequences were aligned to a data set previously obtained from French localities. 4 Tomicus destruens was found in all populations, except for one locality in Portugal and in the Landes (France). It was in sympatry with T. piniperda in two locations on Pinus pinaster and one location on Pinus radiata . 5 Within-population genetic diversity was high, but we found a significant pattern of spatial distribution of genetic variation, as well as a significant effect of the host tree. 6 The data suggest the existence of two glacial refugia, from which T. destruens recolonized its current range. One refugium was located in Portugal where the beetle probably evolved on P. pinaster . The corresponding haplotypes show a West -East frequency gradient. The other refugium was probably in the eastern range, where the beetles evolved on Pinus halepensis and P. pinea . The corresponding haplotypes show an East -West frequency gradient.

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interpretadas com base na análise palinológica de um perfil sedimentar amostrado na vereda da FazendaUrbano. A análise palinológica foi realizada em sete amostras sedimentares ao longo de 60 cm de uma fáciessilto-argilosa com matéria orgânica, datada na base em 13.120 ± 60 anos AP, e consistiu de duas etapas, umaqualitativa, onde foram identificados 51 palinomorfos; e uma quantitativa, cujos resultados foram expressosem formas de diagramas de concentração e porcentagem. A identificação e a contagem dos palinomorfospermitiram caracterizar duas palinozonas (FZU 1 e FZU 2), que refletem variações nos padrões de umidade ena dinâmica da vegetação na região. A palinozona FZU 1 (13.120 ± 60 a 11.640 ± 60 anos AP) foi caracterizadapelo predomínio de grãos de pólen de elementos arbustivos-herbáceos, pela rara ocorrência de grãos depólen de elementos arbóreos e pela ausência de Mauritia flexuosa, sugerindo que formações campestressemelhantes à fitofisionomia Campo Limpo ocupavam a região sob condições climáticas mais secas do queas atuais. Na palinozona FZU 2 (6.000 – idade interpolada a 1.500 ± 40 anos AP) foi observado o aumentode grãos de pólen de elementos arbóreos e na diversidade dos táxons arbustivo-herbáceos e arbóreos, alémda substituição do tipo fitofisionômico Campo Limpo por um mosaico das fitofisionomias Campo Sujo eCerrado sentido restrito, e do estabelecimento da vereda sob condições climáticas mais úmidas, semelhantese/ou comparáveis às condições climáticas atuais da região.Palavras chave: Palinologia, Quaternário, Cerrado, Minas Gerais, vereda ABSTRACT: Palaeoenvironmental and palaeoecological changes since the late Pleistocene in Buritizeiro (MG) wereinterpreted based on pollen analysis of a sedimentary profile collected in a palm swamp from Fazenda Urbano.The pollen analysis was performed in seven sedimentary samples along 60 cm of a silt-clay facies with organicmatter, dated on the basis of 13.120 ± 60 years BP. It was consisted of two steps, a qualitative, whereby 51palynomorphs were identified; and a quantitative, being the results expressed through concentration andpercentage diagrams. The identification and the counting of palynomorphs characterized two palinozones(FZU 1 and FZU 2) that reflect variations in moisture patterns and vegetation dynamics in the region. Thepalinozone FZU 1 (13,120 ± 60 to 11,640 ± 60 years BP) was characterized by the predominance of shrubsand herbs pollen, by the rare occurrence of tree pollen grains and by the absence of Mauritia flexuosa,suggesting that grassland formations similar to Campo Limpo vegetation occupied the region under climateconditions that were drier than today. In palinozone FZU 2 (6.000 - interpolated age to 1.500 ± 40 years BP)it was observed an increase in tree pollen grains and in the diversity of shrubby-herbaceous and trees taxa,besides the replacement of Campo Limpo vegetation by a mosaic of Campo Sujo and Cerrado sensu strictovegetation types and the establishment of the palm swamp under wetter climate conditions, similar and / orcomparable to current weather conditions in the region.Keywords: Palynology, Quaternary, Cerrado, Minas Gerais, palm swamp.