Showing papers by "Matthias Stöck published in 2011"

Ever-Young sex chromosomes in European tree frogs

Abstract: 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.

Is Chytridiomycosis an Emerging Infectious Disease in Asia

Abstract: The disease chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd), has caused dramatic amphibian population declines and extinctions in Australia, Central and North America, and Europe. Bd is associated with >200 species extinctions of amphibians, but not all species that become infected are susceptible to the disease. Specifically, Bd has rapidly emerged in some areas of the world, such as in Australia, USA, and throughout Central and South America, causing population and species collapse. The mechanism behind the rapid global emergence of the disease is poorly understood, in part due to an incomplete picture of the global distribution of Bd. At present, there is a considerable amount of geographic bias in survey effort for Bd, with Asia being the most neglected continent. To date, Bd surveys have been published for few Asian countries, and infected amphibians have been reported only from Indonesia, South Korea, China and Japan. Thus far, there have been no substantiated reports of enigmatic or suspected disease-caused population declines of the kind that has been attributed to Bd in other areas. In order to gain a more detailed picture of the distribution of Bd in Asia, we undertook a widespread, opportunistic survey of over 3,000 amphibians for Bd throughout Asia and adjoining Papua New Guinea. Survey sites spanned 15 countries, approximately 36° latitude, 111° longitude, and over 2000 m in elevation. Bd prevalence was very low throughout our survey area (2.35% overall) and infected animals were not clumped as would be expected in epizootic events. This suggests that Bd is either newly emerging in Asia, endemic at low prevalence, or that some other ecological factor is preventing Bd from fully invading Asian amphibians. The current observed pattern in Asia differs from that in many other parts of the world.

A cryptic heterogametic transition revealed by sex-linked DNA markers in Palearctic green toads.

Abstract: In sharp contrast to birds and mammals, most cold-blooded vertebrates have homomorphic (morphologically undifferentiated) sex chromosomes. This might result either from recurrent X-Y recombination (occurring e.g. during occasional events of sex reversal) or from frequent turnovers (during which sex-determining genes are overthrown by new autosomal mutations). Evidence for turnovers is indeed mounting in fish, but very few have so far been documented in amphibians, possibly because of practical difficulties in identifying sex chromosomes. Female heterogamety (ZW) has long been established in Bufo bufo, based on sex reversal and crossing experiments. Here, we investigate a sex-linked marker identified from a laboratory cross between Palearctic green toads (Bufo viridis subgroup). The F1 offspring produced by a female Bufo balearicus and a male Bufo siculus were phenotypically sexed, displaying an even sex ratio. A sex-specific marker detected in highly reproducible AFLP genotypes was cloned. Sequencing revealed a noncoding, microsatellite-containing fragment. Reamplification and genotyping of families of this and a reciprocal cross showed B. siculus to be male heterogametic (XY) and suggested the same system for B. balearicus. Our results thus reveal a cryptic heterogametic transition within bufonid frogs and help explain patterns of hybrid fitness within the B. viridis subgroup. Turnovers of genetic sex-determination systems may be more frequent in amphibians than previously thought and thus contribute to the prevalence of homomorphic sex chromosomes in this group.

Influence of environmental conditions on the distribution of Central Asian green toads with three ploidy levels

Abstract: We studied the distribution of Palearctic green toads (Bufo viridis subgroup), an anuran species group with three ploidy levels, inhabiting the Central Asian Amudarya River drainage. Various approaches (one-way, multivariate, components variance analyses and maximum entropy modelling) were used to estimate the effect of altitude, precipitation, temperature and land vegetation covers on the distribution of toads. It is usually assumed that polyploid species occur in regions with harsher climatic conditions (higher latitudes, elevations, etc.), but for the green toads complex, we revealed a more intricate situation. The diploid species (Bufo shaartusiensis and Bufo turanensis) inhabit the arid lowlands (from 44 to 789 m a.s.l.), while tetraploid Bufo pewzowi were recorded in mountainous regions (340-3492 m a.s.l.) with usually lower temperatures and higher precipitation rates than in the region inhabited by diploid species. The triploid species Bufo baturae was found in the Pamirs (Tajikistan) at the highest altitudes (2503-3859 m a.s.l.) under the harshest climatic conditions.

Thirteen polymorphic microsatellite markers for the European green toad Bufo viridis viridis, a declining amphibian species

Abstract: We report 13 new polymorphic microsatellite markers for the European green toad Bufo viridis viridis (B. viridis subgroup), a declining amphibian from Central, Southeastern and Eastern Europe. Diversity at these loci estimated for 19 individuals ranged from two to ten alleles. Most of these primers also cross-amplify in related West-Mediterranean green toad species (Bufo balearicus, B. siculus and B. boulengeri). These microsatellites will be useful for conservation genetics of threatened Bufo viridis viridis populations and evolutionary studies of green toad taxa in secondary contact to examine hybridization.

Polymorphism at a Sex-Linked Transcription Cofactor in European Tree Frogs (Hyla arborea): Sex-Antagonistic Selection or Neutral Processes?

Abstract: Nascent sex chromosomes offer a unique opportunity to investigate the evolutionary fate of genes recently trapped in non-recombining segments A house-keeping gene (MED15) was recently shown to lie on the nascent sex-chromosomes of the European tree frog (Hyla arborea), with different alleles fixed on the X and the Y chromosomes Here we document a polymorphism (glutamine deletion) in the X copy of the gene, and use population surveys and experimental crosses to test whether this polymorphism is neutral or maintained by sex-antagonistic selection Tadpoles from parents of known genotypes revealed significant discrepancies from Mendelian inheritance, suggesting possible sex-antagonistic effects under laboratory conditions Quantitatively, however, these effects did not meet the conditions for polymorphism maintenance Furthermore, field estimates of female genotypic frequencies did not differ from Hardy–Weinberg equilibrium and allelic frequencies on the X chromosome did not differ between sexes In conclusion, although sex-antagonistic effects cannot be excluded given the laboratory conditions, the X-linked polymorphism under study appears neutral in the wild Alternatively, sex-antagonistic selection might still account for the fixation of a male-specific allele on the Y chromosome

The Mating Call and Male Release Call of Bufo raddei Strauch, 1876 with Some Phylogenetic Implications

Abstract: The mating call of Bufo raddei is analyzed and described for the first time. Although species-specific, important parameters (call duration, amplitude-time waveform of single pulses, absence of interpulse intervals at higher body temperatures) of the mating call show a similar structure to those of the allopatric Bufo calamita , mainly differing by lower pulse rates. B. raddei mating calls differ from those of diploid, triploid and tetraploid representatives of the parapatric Eurasian Bufo viridis complex which have a pulsed structure with distinct interpulse intervals. The release call of male B. raddei consists of a single pulse group with slight frequency modulation and appears similar to the release call of B. calamita , but differs from those in the B. viridis complex which always are subdivided in pulse groups. We suggest the occurrence of two different types of amplitude modulation as an indication for a possible divergence of two evolutionary lineages, the B. calamita / B. raddei lineage and the B. viridis complex lineage. Furthermore, we compare these results with other published phylogenetic data on these taxa.

Stripeless tree frogs (Hyla meridionalis) with stripes on the Canary Islands

Abstract: Anurans are known to exhibit a wide array of colour and pattern polymorphisms, including “background” colour and the presence or absence of stripes and spots. Most studies suggest that these traits are heritable (Hoffman & Blouin 2000, O’Neill & Beard 2010), which means that variations can be shaped and influenced by drift, and pred-atory and sexual selection (e.g., Taylor et al. 2007), in ad-dition to other factors, such as ontogenetic changes, sexu-al dimorphism, and physiological plasticity (Hoffman & Blouin 2000). In hylid frogs, an array of physiological col-our changes has been documented (e.g., Schneider 2009 and references therein). The complex processes that main-tain colour polymorphisms in nature allow displaying a large diversity of phenotypes.Here we focus on the stripeless tree frog