Abstract
Morphological and chromosomal characteristics of a number of unionid freshwater mussels were studied from northeastern Thailand. Karyotypes of eight species from seven genera (Chamberlainia, Ensidens, Hyriopsis, Physunio, Pseudodon, Scabies and Trapezoideus) were examined. Six species possess 2n = 38 karyotypes, whereas Scabies crispata and an unidentified Scabies sp. lack three small chromosome pairs, giving a diploid number of 32. Moreover, the karyotypes of the unidentified Scabies differ from Scabies crispata as it exhibits a telocentric chromosome pair (6m + 7sm + 2st + 1t). Most of the conchological characters also differ between the two species – adult size, colour pattern, muscle scars, pseudocardinal and lateral teeth. The name Scabies songkramensis sp. n. is proposed for the unidentified species, and its description is included in this paper. Interestingly, seven species contain mostly bi-armed chromosomes, but only the mud-dweller in stagnant water, Ensidens ingallsianus, contains predominantly five telocentric pairs. In addition, the marker chromosome characteristics of an unbalanced long arm, twisted centromere, a wider angle 180° arrangement, a twisted arm and telomeric end union reported in this study are described for the first time for unionid mussels.
Highlights
The Unionidae is numerically the largest family of both extant and extinct freshwater mussels and includes over 670 species worldwide with about 220 species occurring in Indotropica (Graf and Cummings 2007)
Metaphase plates in which the chromosomes were clearly differentiated within the cells were selected for study
The diploid numbers of six species in the three subfamilies, Hyriopsinae, Pseudodontinae and Rectidentinae, showed the same chromosome number (2n = 38), which is similar to unionid taxa in other regions (Vitturi et al 1982; Meesukko 1996; Jenkinson 2014; see Table 2)
Summary
The Unionidae is numerically the largest family of both extant and extinct freshwater mussels and includes over 670 species worldwide with about 220 species occurring in Indotropica (Graf and Cummings 2007). Recent reassessments by other malacologists have revealed some new and unknown species (Graf 2002; Deein et al 2003) and there are still many localities that have never been surveyed Owing to their conservative morphological diversity, it is has not been easy to establish a reliable phylogeny for unionids. Tests of phylogenetic hypotheses on the basis of other data sources, such as those derived from molecules and chromosomes, are likely to be informative Such approaches have as yet been attempted only on a limited number of taxa and there are still very few studies in Asian and African regions (Lopes-Lima et al 2014; Marshall et al 2014; Graf et al 2015)
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