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

Abstract

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 6^th, 7^th or 10^th 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 6^th 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 6^th 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).

Keywords: Amphibia; Madagascar; NORs; chromosome evolution; karyotype.

Figure 1.

Giemsa stained karyotypes of AGephyromantisstriatus (FN 7645) BMantidactylus (Brygoomantis) alutus (FN 7945) CSpimomantispropeaglavei “North” (FN 7543) DGephyromantis sp. Ca19 (FN 7630) EMantidactylus (Chonomantis) sp. Ca11 (FN 7545) FMantidactylus (Hylobatrachus) cowanii (FAZC 11370) GSpinomantis sp. Ca3 (FN 7567) and HSpinomantisphantasticus (FG/MV 2002-970). Insets represent NOR-bearing pairs stained with Giemsa (down in the insets) and Ag-NOR method (up in the insets).

Figure 2.

Metaphase plates of Gephyromantisstriatus (A, A’, A”), Mantidactylus (Brygoomantis) alutus (B, B’, B”), Spinomantispropeaglavei “North” (C, C’, C”), Gephyromantis sp. Ca19 (D, D’, D”), Spinomantis sp. Ca3 (E, E’, E”) and Mantidactylus (Hylobatrachus) cowanii (F, F’, F”) stained with C-banding + Giemsa (A–F), + CMA (A’–F’) + DAPI (A”–F”). Arrows point at NORs while arrowheads highlight other heterochromatin blocks.

Figure 3.

Hypothesized general model of chromosome reduction in Mantellinae from n = 13 (2n = 26) to n = 12 (2n = 24) by means of chromosome fusions. Red dots highlight the NOR bearing chromosome.