Cytogenetic characterisation of the razor shells Ensis directus (Conrad, 1843) and E. minor (Chenu, 1843) (Mollusca: Bivalvia)

Abstract

The European razor shell Ensis minor (Chenu 1843) and the American E. directus (Conrad 1843) have a diploid chromosome number of 38 and remarkable differences in their karyotypes: E. minor has four metacentric, one metacentric-submetacentric, five submetacentric, one subtelocentric and eight telocentric chromosome pairs, whereas E. directus has three metacentric, two metacentric-submetacentric, six submetacentric, six subtelocentric and two telocentric pairs. Fluorescent in situ hybridisation (FISH) using a major ribosomal DNA probe located the major ribosomal genes on one submetacentric chromosome pair in both species; FISH with a 5S ribosomal DNA (5S rDNA) probe rendered one chromosomal (weak) signal for E. minor and no signal for E. directus, supporting a more dispersed organisation of 5S rDNA compared to the major ribosomal genes. The vertebrate telomeric sequence (TTAGGG) n was located on both ends of each chromosome, and no interstitial signals were detected. In this work, a comparative karyological analysis was also performed between the four Ensis species analysed revealing that the three European species studied so far, namely E. minor, E. siliqua (Linné 1758) and E. magnus Schumacher 1817 show more similarities among them than compared to the American species E. directus. In addition, clear karyotype differences were found between the morphologically similar species E. minor and E. siliqua. Keywords Razor shells Á Karyotype Á FISH Á 18S-5.8S-28S rDNA Á 5S rDNA Á Telomeric sequence ''E. minor was frequently treated under the name E. siliqua or as a subspecies of that taxon; however, along the Atlantic coast the two species occur sympatrically with Communicated by Heinz-Dieter Franke.

Are Pericentric Inversions Reorganizing Wedge Shell Genomes?

“…The 2n = 38 diploid chromosome numbers of the two wedge shell species studied here coincide with previous reports for Donax variabilis [ 23 ] and D. trunculus [ 21 , 24 , 25 ]. This is also the usual diploid number found in most bivalve species belonging to the subclass Heterodonta [ 27 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 ].…”

“…In contrast, the karyotypes of the wedge shells studied here present some unusual characteristics for the subclass Heterodonta. Typically, those bivalve karyotypes are poor in telocentric chromosomes [ 41 , 44 ] and the presence of undetectable short arms in the telocentric pairs is uncommon. The best known examples are the dwarf surf clam Mulinia lateralis [ 45 ], three razor shell species [ 41 , 46 ], and one tellin shell species [ 44 ].…”

“…Typically, those bivalve karyotypes are poor in telocentric chromosomes [ 41 , 44 ] and the presence of undetectable short arms in the telocentric pairs is uncommon. The best known examples are the dwarf surf clam Mulinia lateralis [ 45 ], three razor shell species [ 41 , 46 ], and one tellin shell species [ 44 ]. This is also the case for D. vittatus karyotypes in which, of a total of 10 or 11 telocentric chromosome pairs, only the 45S rDNA-bearing chromosome pair 6 clearly presented detectable short arms.…”

Species delimitation and DNA barcoding of Atlantic Ensis (Bivalvia, Pharidae)

“…Our results agree with the taxonomic revision by Cosel () based on shell morphology and with González‐Tizón et al . () who found important differences among the karyotypes of both species.…”

“…Interestingly, González‐Tizón et al . () supported both taxa as separate species according to cytogenetics.…”