Sex Determination by a Univalent Chromosome in the Rhizocephalan Peltogasterella gracilis (Cirripedia: Rhizocephala: Peltogasterellidae).

AbstractKentrogonid rhizocephalans show remarkable sexual dimorphism, with large females and much smaller (=dwarf) males as adults, although males are larger than females from the eggs to the larval stages. The kentrogonid Peltogasterella gracilis shows seasonal variation in sex ratio in that most females produce only male or female offspring, although some mixed-sex broods appear in autumn. Moreover, a link between chromosome number and the single-sex broods has been suggested, wherein females producing only male offspring are considered to have a chromosome number of 2n=30, whereas females producing only female offspring are 2n=31. Thus, there appear to be both genetic and environmental effects on variation in sex ratio. However, the mechanism of sex determination has not been fully studied in P. gracilis or any other rhizocephalans. We examined the number of chromosomes in male (larger) and female (smaller) unfertilized eggs and early-stage embryos of specimens collected in Otaru, Hokkaido, Japan. Chromosome number of unfertilized eggs was n=16 for female eggs and n=15 for male eggs, without exception. Moreover, chromosome number in most (97.4%) female embryos was 2n=31, while that of all male embryos was 2n=30. We suggest that a univalent chromosome of females acts as a sex chromosome and P. gracilis has a special type of female heterogamety (OW/OO system). This sex determination system appears to be suitable for controlling offspring sex ratio seasonally, as is known in this rhizocephalan.