SINGLE COPY DNA HOMOLOGIES IN ATRIPLEX .2. HYBRID THERMAL STABILITIES AND MOLECULAR PHYLOGENY

Single copy DNA sequence homologies were measured by interspecific molecular hybridization and thermal denaturation techniques for 8 spp. of Atriplex [A. serenania, A. fruticulosa, A. rosea, A. sabulosa, A. phyllostegia, A. hortensis, A. truncata and A. triangularis] and spinach (Spinacia oleracea). Thermal stability profiles for Atriplex interspecific hybrids indicated more base pair mismatch than has been observed in most previously reported intrageneric comparisons of animal DNA. On the assumption that sequence divergence by base substitution is proportional to the time which has elapsed subsequent to speciation, the data are interpreted as indicating that lines leading to many modern Atriplex spp. probably originated during a single period of rapid speciation. This group includes C3 and C4 photosynthetic types from both subgenera. Thus the molecular data do not support the classical subgeneric distinction, and it is no longer necessary to postulate a polyphyletic origin for C4 photosynthesis within Atriplex.