ALLELIC VARIATION IN THE AMPHITROPICAL DISJUNCT MUHLENBERGIA-TORREYI (POACEAE, MUHLENBERGIINAE)

Muhlenbergia torreyi occurs in the southwestern United States, norther n? Mexico, and northwestern Argentina. Allozyme data were used to eval uate genetic diversity within and among populations of this amphitropi cal disjunct species. Electrophoretic examination of 22 putative enzym e loci in 15 populations revealed high levels of genetic variation (P ranging from 64.7 to 82.4; H from 0.527 to 0.757) and high levels of g enetic diversity (F ranging from -0.584 to -0.939). All populations po ssess high levels of heterogeneity (F,, approaching -1, mean of -0.837 ) and exhibit lower levels of genetic fixation among populations (F-ST mean of 0.127). A comparison of genetic identity values among populat ions from North and South America indicates that the genetic variation is slightly greater (I = 0.93) in North America than in South America (I = 0.96). A total of 51 alleles were shared among all populations, and four unique alleles were detected: two from North American populat ions and two from South America. It seems likely that Muhlenbergia tor reyi has dispersed to South America recently, because the populations there are less variable. Chromosome numbers, not determined by the aut hors, of 2n = 20, 21 for M. torreyi and 2n = 80, 82 for M. arenicola a re reported. Muhlenbegia arenicola is first reported from South Americ a.