GENOME SIZE AND KARYOTYPE LENGTH IN SOME INTERSTITIAL POLYCHAETE SPECIES OF THE GENUS OPHRYOTROCHA (DORVILLEIDAE)

Interstitial polychaetes of the genus Ophryotrocha are very small, pro genetic, and morphologically very similar. These worms have been widel y used in evolutionary biology and sexuality studies. To have a better insight into the karyological evolution of this genus, we measured th e total karyotypic length and the 2C nuclear DNA content of the nine b est-known species of this genus. No inters specific differences were o bserved in karyotypic lengths, apart from that of 0. gracilis, which w as significantly greater than the karyotypic length of five of the nin e species. The genome size (i.e., 1C DNA content calculated from 2C DN A content) in eight of the nine species is about 0.4 pg, irrespective of the chromosome number. A group of four gonochoric and morphological ly indistinguishable species, with 2n = 6 metacentric chromosomes, app ears to be heterogeneous with regard to its DNA content, because one o f the species, 0. macrovifera, has a genome twice the size of that of the other three species. A hermaphroditic species, 0. hartmanni, has a genome three times that size. No correlation has been observed betwee n genome size and body size, egg cell diameter, or time interval from egg fertilization to sexual maturity. The basic genome size of 0.4 pg is among the lowest recorded in invertebrates. Hypotheses about select ive pressures that maintain such a low amount of nuclear DNA in this g enus are discussed.