MOLECULAR DELINEATION OF SPECIES AND SYNGENS IN VOLVOCACEAN GREEN-ALGAE (CHLOROPHYTA)

Two species of the colonial flagellate family Volvocaceae are worldwid e in distribution yet exhibit contrasting species structure. Geographi cally disparate isolates of Gonium pectorale Mueller can interbreed wh ile isolates of Pandorina morum Bory behave quite differently, More th an 20 sexually isolated subpopulations occur within this species; thes e have been termed ''syngens'' (sensu Sonneborn). Because prezygotic b arriers to mating cause intersyngen pairings to fail, breeding analyse s cannot be used to estimate genetic relatedness among the syngens of P. morum. DNA comparisons provide an alternative method of assessing g enetic relatedness. We compared the nucleotide sequence of the interna l transcribed spacer (ITS) region of the nuclear ribosomal repeat amon g clones of P. morum and of G. pectorale. Members of syngens of P. mor um with distribution restricted to one small geographical area show gr eat similarity. Likewise, members of any syngen of worldwide distribut ion show near uniformity, even those from different continents. Howeve r, the ITS sequence of each syngen differs from that of other syngens. In contrast, G. pectorale, which has an ITS region that is remarkably uniform throughout the world, appears to consist of a single syngen w ithin North America and Europe by mating tests. The molecular data are in complete conformity with previous syngen assignment. Because the l atter is based on mating affinity, with two complementary mating types per syngen, the evolution of new mating type pairs appears to be the basis of microevolution in these algae. We infer that either P. morum is a more ancient species than G. pectorale or that P. morum has a les s stable genome. In either case, the biogeographic distribution of cer tain syngens may reflect climatological changes of the past.