Cytogenetic Patterns of Evolutionary Divergence in the Mimulus glabratus Complex

Abstract

Diploid (n = 15) populations of the Mimulus glabratus complex have radiated out over much of North America without loss of chromosomal homology. They have accumulated numerous morphologi- cal differences and partial-to-complete barriers to gene exchange that subdivide the complex into at least eight subgroups. Eutetraploids (n=30), which range S of the diploids, share the same basic genome but have evolved at least four different second genomes, each of which occurs in a morphologically distinct and genetically isolated population or popu- lation cluster, i.e., species. Aneuploid tetraploids (n=31) have radiated out in Central America retaining the basic and second genomes but with the accumulation of partial-to-complete barriers to gene exchange with or without morphological differences. They form four subgroups that range from weakly marked varieties to a sibling species. Hexaploids (n =46), widespread in South America, appear to form two subgroups which contain the basic genome but which have lost chromosome homol- ogy in the other genomes with a corresponding barrier to gene exchange between the two subgroups, probably species. The large Chilean sub- group has differentiated morphologically without any concomitant chro- mosomal differentiation or any accumulation of barriers to gene exchange. Thus, each heteroploid level exhibits its own characteristic evolutionary pattern. It is suggested that the diploids produced the eutetraploids and aneuploid tetraploids by autopolyploidy and the subsequent differential diploidization of the second genomes. Further, it is postulated that in the area where these forms overlap in northern Mexico, an n = 15 x n = 31 alloploid arose, was transported by migratory birds to South America and there radiated out as the hexaploid group of the complex. The evi- dence presented is consistent with this putative phylogeny.