Abstract
Wedge shells belonging to the Donacidae family are the dominant bivalves in exposed beaches in almost all areas of the world. Typically, two or more sympatric species of wedge shells differentially occupy intertidal, sublittoral, and offshore coastal waters in any given locality. A molecular cytogenetic analysis of two sympatric and closely related wedge shell species, Donax trunculus and Donax vittatus, was performed. Results showed that the karyotypes of these two species were both strikingly different and closely alike; whilst metacentric and submetacentric chromosome pairs were the main components of the karyotype of D. trunculus, 10–11 of the 19 chromosome pairs were telocentric in D. vittatus, most likely as a result of different pericentric inversions. GC-rich heterochromatic bands were present in both species. Furthermore, they showed coincidental 45S ribosomal RNA (rRNA), 5S rRNA and H3 histone gene clusters at conserved chromosomal locations, although D. trunculus had an additional 45S rDNA cluster. Intraspecific pericentric inversions were also detected in both D. trunculus and D. vittatus. The close genetic similarity of these two species together with the high degree of conservation of the 45S rRNA, 5S rRNA and H3 histone gene clusters, and GC-rich heterochromatic bands indicate that pericentric inversions contribute to the karyotype divergence in wedge shells.
Highlights
Genomic data obtained in the last two decades have revealed that chromosomes display a plethora of reorganizations [1,2]
As part of our ongoing bivalve cytogenetic research program, we found that in another species of wedge shell, Donax vittatus, the metaphase plates contained a high proportion of chromosome pairs with hardly detectable short arms, i.e., telocentric chromosomes
The nucleotide sequences for all wedge shell specimens morphologically assigned to D. trunculus (GenBank accession numbers KY951431 to KY951446)
Summary
Genomic data obtained in the last two decades have revealed that chromosomes display a plethora of reorganizations [1,2]. The patterns of chromosomal rearrangements are, in many cases, lineage-specific [3]. Some wedge shell specimens are identifiable according to shell morphology, others can only be recognized after studying shell characteristics, which exhibit a great variability, and intermediate states between taxa are often observed [9,10]. This is further complicated by the presence of two or more geographically-sympatric species of Donax differentially occupying intertidal, sublittoral, and offshore regions in almost any given locality [6]
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