Chromosomal Evolution in North American Catfishes (Siluriformes: Ictaluridae) with Particular Emphasis on the Madtoms, Noturus

Abstract

has 2N = 56; species currently placed in Ictalurus have between 48 (I. catus) and 62 (I. natalis and I. brunneus) with diploid numbers of 52, 54, 56, 58 and 60 present in one or more species. The members of the subgenus Amiurus have diploid numbers either greater than or less than 58, while both I. (Ictalurus) punctatus and I. furcatus have 2N = 58. An ancestral ictalurid karyotype of 2N = 58 with a relatively high FN (>80) is postulated, based primarily on the distribution of chromosomal states in the various ictalurid lineages as well as in peripherally related siluriform families (Bagridae, Pimelodidae and Ariidae). This karyotype may be ancestral for all siluriforms. Ictalurids (and siluriforms in general) have karyotypes characterized by many chromosomes with identifiable second arms (ranging from minute to large). Robertsonian rearrangements, including primarily centric fusions but also centric dissociations, appear to have been important in the evolution of ictalurid karyotypes. Interspecific karyotype differences cannot be totally explained as Robertsonian events; additional rearrangements such as pericentric inversions must be invoked to explain the karyotypic differences. There has been parallel evolution of the karyotypes within madtoms; there has been stepwise reduction of 2N from 54 to 42 in Schilbeodes and 52 to 40 in Rabida. The hypothetical common ancestor of the Noturus-Pylodictis line probably possessed a karyotype similar to that in Pylodictis with a 2N = 56 karyotype with a high FN (>80). Early stages of Noturus divergence may have involved initial reduction of the 2N to 54 with a high FN retained; such a karyotype is seen in Noturus gilberti (2N = 54, FN = 82) and is closely approached by Noturus flavipinnis with 2N = 52, FN = 82. Karyotypic data available provide little new insight into intergeneric relationships or intrageneric relationships in Ictalurus. Chromosomal data do, however, provide the basis for a new hypothesis on intrageneric relationships within Noturus. Relationships suggested by chromosomal analysis are consistent with some relationships postulated previously while differing in other respects. Emphasis is placed on the transformation of diploid numbers since difficulties are encountered in determination of accurate fundamental numbers due to the large number of chromosomes that possess second arms and the gradual change in the size and form of the elements in most karyotypes. Chromosomal similarities to the subgenus Schilbeodes as well as the lack of sufficiently unique morphological characters suggest the consolidation of the currently recognized subgenus Noturus with Schilbeodes.