MATING SYSTEM-ANALYSIS USING PROTEIN ELECTROPHORESIS IN THE SELF-FERTILE HERMAPHRODITE SPECIES BULINUS-TRUNCATUS (GASTROPODA, PLANORBIDAE)

Uniparental reproduction in the tetraploid hermaphrodite species Bulin us truncatus has been suggested to occur via self-fertilization, on th e basis of cytological and genetic studies. However, recent analyses o f population genetic structure by protein electrophoresis indicated th e occurrence of multibanded phenotypes referred to as 'fixed heterozyg osity'. Although intrapopulation and geographical variation of multiba nded phenotypes occur, many populations bear only one type of such pat terns. This led to the suggestion that parthenogenesis may well be the mating system in B. truncatus. However, such fixed heterozygosity pat terns are expected in some tetraploid species in which double disomic inheritance of alleles occurs. It is therefore not possible to determi ne from population genetic structure analysis alone whether segregatio n occurs or not. Here, we investigate the stability of such patterns o ver one generation of uniparental reproduction among four populations. We also present resultS of crossing experiments between individuals f rom two populations, using two diagnostic loci to analyse their offspr ing. Our results clearly indicate Mendelian segregation of alleles, an d confirm sexual reproduction by self-fertilization and cross-fertiliz ation. We interpret the multibanded patterns observed in populations a s the product of both diploid loci of the tetraploid genome when they are monomorphic for different alleles. Our study also allows us to sug gest that partial selfing may be the regular mating system in B. trunc atus.