LEUCISCUS (PISCES, CYPRINIDAE) KARYOTYPES - TRANSECT OF PORTUGUESE POPULATIONS

The presently described Iberian chubs - Leuciscus carolitertii and L. pyrenaicus - sampled throughout their distribution ranges in Portugal were cytogenetically analyzed. Their chromosome numbers were consisten tly 2n = 50, except for two specimens of L, carolitertii, which exhibi ted a supernumerary chromosome in some of the metaphases. The karyotyp es were found to be highly typical for other Leuciscus taxa, as well a s for European leuciscine cyprinids: the chromosome sets are dominated by metacentric and submetacentric elements with a reduced number of a crocentric pairs (three to four); the largest pair of the complements belongs typically to this latter category. The chubs from northern dra inages, assignable to L. carolitertii, have apparently a more stable k aryotype structure (12M:30S:8A) than the chubs from L. pyrenaicus, whi ch have 12M:32S:6A, but may exhibit in the most southern river basins (Guadiana, Mira, Aljezur, Bordeira and Arade) more variable karyotypes . Besides, these data support the very recent discovery of two genetic ally distinct Leuciscus taxa in this region of the Iberian Peninsula, suggesting the stochastic fixation of structural chromosome rearrangem ents in these small and isolated drainages, which may be affected by b ottlenecks due to significant variations in hydrological regimes. The NORs were apparently located in one small submetacentric Fair of chrom osomes and the presence of a heteromorphic sex chromosome system of th e ZW/ZZ type was also evidenced for the Iberian endemic chubs.