Biogeography and speciation in the Pandorina/Volvulina (Chlorophyta) superclade

Mating affinity, nuclear rDNA internal transcribed spacer (ITS) sequence, a nd geographic distribution of more than 100 isolates of Pandorina,Volvulina , and Yamagishiella were determined. Comparative analysis of ITS sequences reaffirmed the unity of Pandorina morum and its separation from all other s pecies of Volvocaceae except those of Volvulina. This latter genus, represe nted by four species, appears to represent only morphological variants of s everal P. morum subclades. The P. morum clade (including also additional sp ecies such as P. smithii and P. colemaniae) encompasses an evolutionary spa n, as determined by comparison of ITS, greater than either of the multispec ies genera Gonium or Eudorina. There are at least 30 Pandorina/Volvulina sy ngens, sexually isolated groups, so far as can be determined, among the cur rent collection of strains. In addition, as in other volvocacaean genera, t wo clones are homothallic, capable of forming zygotes within a genetic clon e. The existence of so many syngens suggests that considerable evolutionary diversification of the genes controlling gamete compatibility and intercro ss survival has occurred, unaccompanied by significant morphological change . Within each syngen, genetic distance increases with geographic distance b etween collection sites. At least half of the isolates studied must have be en introduced northward since the Pleistocene. Although we probably know mo re about characters in this group of algae than any other algal group, in p art because they are so easy to culture, we are still largely ignorant of w hat circumscribes their niche in nature. The study of all these organisms, distributed throughout the world presumably by the activities of shorebirds , emphasizes the evolutionary role of mating genes in the inevitable format ion of new genetically isolated subclades in these eukaryotes and provides initial data on their rate of appearance versus their rate of distribution over the earth.