DOES POLYPLOIDY LEAD TO FEWER AND SHORTER MICROSATELLITES IN BARBUS (TELEOSTEI, CYPRINIDAE)

Screening of a hybrid Barbus barbus-B. meridionalis genome was perform ed for CA, GA, TAT, TCT, TAG, TGT, TATT, TACT, ATCT motifs, and simult aneously on another fish species, tilapia S. melanotheron. Sequences o f positive clones were obtained for Barbus and revealed that repetitiv e structure significantly depends on the motif: most TAT and TATT repe ats contain small numbers of repeats, and these repeats are highly het erogeneous, whereas other motifs (we mainly obtained CA and GATA repea ts) form longer and much more homogeneous arrays. Polymorphism data fr om five loci in two different species of barbel show that perfectly re petitive loci are much more variable than imperfect loci (TAT and TATT ). We compared the frequency of positive clones for different repeat m otifs between barbel and tilapia. For dinucleotide repeats (CA and GA) , the comparison was extended to additional fish species, trout and se a bass, which were screened in nearly identical conditions for these m otifs. The most salient feature of these comparisons reveals that arra ys of dinucleotide motifs are significantly under-represented and shor ter in Barbus than in other fish species. We propose an explanation th at can account for most features of microsatellites characterizing the genome of barbel. A bias toward deletion affecting slipped-strand mis pairing events would lead to shortening and loss of microsatellite loc i. Such a bias would represent an efficient way of eliminating useless DNA from polyploidized species with an excessive amount of DNA.