Evidence for a remarkable stasis of chromosome evolution in Malagasy treefrogs (Boophis, Mantellidae)

Comparative cytogenetics on eight Malagasy Mantellinae (Anura, Mantellidae) and a synthesis of the karyological data on the subfamily

We performed a molecular and cytogenetic analysis on different Mantellinae species and revised the available chromosomal data on this group to provide an updated assessment of its karyological diversity and evolution. Using a fragment of the mitochondrial 16S rRNA, we performed a molecular taxonomic identification of the samples that were used for cytogenetic analyses. A comparative cytogenetic analysis, with Giemsa's staining, Ag-NOR staining and sequential C-banding + Giemsa + CMA + DAPI was performed on eight species: Gephyromantis sp. Ca19, G.striatus (Vences, Glaw, Andreone, Jesu et Schimmenti, 2002), Mantidactylus (Chonomantis) sp. Ca11, M. (Brygoomantis) alutus (Peracca, 1893), M. (Hylobatrachus) cowanii (Boulenger, 1882), Spinomantispropeaglavei "North" (Methuen et Hewitt, 1913), S.phantasticus (Glaw et Vences, 1997) and S. sp. Ca3. Gephyromantisstriatus, M. (Brygoomantis) alutus and Spinomantispropeaglavei "North" have a karyotype of 2n = 24 chromosomes while the other species show 2n = 26 chromosomes. Among the analysed species we detected differences in the number and position of telocentric elements, location of NOR loci (alternatively on the 6th, 7th or 10th pair) and in the distribution of heterochromatin, which shows species-specific patterns. Merging our data with those previously available, we propose a karyotype of 2n = 26 with all biarmed elements and loci of NORs on the 6th chromosome pair as the ancestral state in the whole family Mantellidae. From this putative ancestral condition, a reduction of chromosome number through similar tandem fusions (from 2n = 26 to 2n = 24) occurred independently in Mantidactylus Boulenger, 1895 (subgenus Brygoomantis Dubois, 1992), Spinomantis Dubois, 1992 and Gephyromantis Methuen, 1920. Similarly, a relocation of NORs, from the putative primitive configuration on the 6th chromosome, occurred independently in Gephyromantis, Blommersia Dubois, 1992, Guibemantis Dubois, 1992, Mantella Boulenger, 1882 and Spinomantis. Chromosome inversions of primitive biarmed elements likely generated a variable number of telocentric elements in Mantellanigricans Guibé, 1978 and a different number of taxa of Gephyromantis (subgenera Duboimantis Glaw et Vences, 2006 and Laurentomantis Dubois, 1980) and Mantidactylus (subgenera Brygoomantis, Chonomantis Glaw et Vences, 1994, Hylobatrachus Laurent, 1943 and Ochthomantis Glaw et Vences, 1994).

Revisiting the Karyotypes of Alligators and Caimans (Crocodylia, Alligatoridae) after a Half-Century Delay: Bridging the Gap in the Chromosomal Evolution of Reptiles

Although crocodilians have attracted enormous attention in other research fields, from the cytogenetic point of view, this group remains understudied. Here, we analyzed the karyotypes of eight species formally described from the Alligatoridae family using differential staining, fluorescence in situ hybridization with rDNA and repetitive motifs as a probe, whole chromosome painting (WCP), and comparative genome hybridization. All Caimaninae species have a diploid chromosome number (2n) 42 and karyotypes dominated by acrocentric chromosomes, in contrast to both species of Alligatorinae, which have 2n = 32 and karyotypes that are predominantly metacentric, suggesting fusion/fission rearrangements. Our WCP results supported this scenario by revealing the homeology of the largest metacentric pair present in both Alligator spp. with two smaller pairs of acrocentrics in Caimaninae species. The clusters of 18S rDNA were found on one chromosome pair in all species, except for Paleosuchus spp., which possessed three chromosome pairs bearing these sites. Similarly, comparative genomic hybridization demonstrated an advanced stage of sequence divergence among the caiman genomes, with Paleosuchus standing out as the most divergent. Thus, although Alligatoridae exhibited rather low species diversity and some level of karyotype stasis, their genomic content indicates that they are not as conserved as previously thought. These new data deepen the discussion of cytotaxonomy in this family.

Description of a new Malagasy treefrog (Boophis) occurring syntopically with its sister species, and a plea for studies on non-allopatric speciation in tropical amphibians

Based on concordant differences in male advertisement call, tadpole morphology, and absence of haplotype sharing in mitochondrial and nuclear DNA markers, we describe a new species of treefrog from Ranomafana National Park in the southern central east of Madagascar. In its adult stage Boophis narinsi sp. n. is highly similar to its sister species, Boophis majori, but appears to differ in having longer hindlimbs. The genetic divergences between these two species (2.5-3.3% in a fragment of the 16S rRNA gene, depending on fragment length and individual haplotype analyzed) are below the threshold typically characterizing distinct species of anurans. Together with their relatively small and largely overlapping ranges and their sympatric occurrence in Ranomafana National Park, this indicates that they potentially could have originated rather recently by adaptive speciation under parapatric or sympatric conditions. Most studies on amphibian speciation have so far by default assumed vicariant speciation. We suggest that alternative speciation scenarios should be considered in future works and characterize settings in which more reliable assessments of adaptive parapatric or sympatric speciation could be carried out.