GENETICS OF SEX DETERMINATION IN FLOWERING PLANTS

Most flowering plant species are hermaphroditic, but a small number of species in most plant families are unisexual (i.e., an individual wil l produce only male or female gametes). Because species with unisexual flowers have evolved repeatedly from hermaphroditic progenitors, the mechanisms controlling sex determination in flowering plants are extre mely diverse. Sex is most strongly determined by genotype in all speci es but the mechanisms range from a single controlling locus to sex chr omosomes bearing several linked loci required for sex determination. P lant hormones also influence sex expression with variable effects from species to species. Here, we review the genetic control of sex determ ination from a number of plant species to illustrate the variety of ex tant mechanisms. We emphasize species that are now used as models to i nvestigate the molecular biology of sex determination. We also present our own investigations of the structure of plant sex chromosomes of w hite campion (Silene latifolia = Melandrium album). The cytogenetic ba sis of sex determination in white campion is similar to mammals in tha t it has a male-specific Y-chromosome that carries dominant male deter mining genes. If one ropy of this chromosome is in the genome, the pla nt is male. Otherwise it is female. Like mammalian Y-chromosomes, the white campion Y-chromosome is rich in repetitive DNA. We isolated repe titive sequences from microdissected Y-chromosomes of white campion to study the distribution of homologous repeated sequences on the Y-chro mosome and the other chromosomes. We found the Y to be especially rich in repetitive sequences that were generally dispersed over all the wh ite campion chromosomes. Despite its repetitive character, the Y-chrom osome is mainly euchromatic. This may be due to the relatively recent evolution of the white campion sex chromosomes compared to the sex chr omosomes of animals. (C) 1994 Wiley-iiss, Inc.