Systematics and evolution of the Carex pachystachya complex (Cyperaceae)

Multivariate statistical analyses of 26 morphological characters indicates the ten previously recognized species in the complex represent seven taxonomic species and two subspecies. One subspecies, C. pachystachya ssp.compacta, is newly recognized. Although breaks in the continuum of the quantitative characters are evident, there still exists significant overlap in many of the characters. Cytological studies on the four chromosomal races (n = 37, 38,39, 41) of C. pachystachya show the n = 41 race comprises the ssp. compacta which occurs in high montane to subalpine regions. The remaining races show no correlation with morphology but do occur in lower elevations. Multiple counts in populations show chromosome number does not vary in populations nor between parents and progeny. Thus populations are fixed for a particular chromosome number. Crossing studies among the chromosome races of C. pachystachya show the plants are self-compatible and prefer inbreeding to outcrossing. All races are cross-compatible, but fertility in the F1 are significantly reduced. Meiotic analysis of F1 progeny shows this results from numerous chromosomal structural rearrangements. The fewest chromosomal differences occur among the n = 37, 38 and 39 races while the n = 41 race shares as many translocation differences with the other races as do related species. This conclusively demonstrates chromosomal repatterning has taken place during the evolution of a perennial plant group. Enzyme electrophoresis within and among 67 populations of the complex results in high average genetic identities both among populations within a species and between populations of different species ({dollar}>{dollar}0.9). Identity values range from a low of 0.6 to a high of 1.0. These ranges result mainly from populations being fixed for different alleles and concur with the crossing studies that populations are inbreed in nature. In total, the data suggest a recent origin of the group. Evolution must have been rapid to allow the morphological variation to have accumulated for the recognition of seven species. Finally, a mechanism for the rapid evolution has been shown by seeing how chromosome number change, combined with chromosomal structural rearrangements and inbreeding allows reproductive isolation to develop rapidly. Isolated populations become fixed for different isozyme alleles, and probably other alleles, resulting in rapid genetic divergence.