THE INDEPENDENT ANEUPLOID ORIGIN OF TWO SPECIES OFCHAENACTIS(COMPOSITAE) FROM A COMMON ANCESTOR

Abstract

Very similar species are of great interest to students of evolution because such species probably have evolved recently, making it likely that much of the evidence of their origin can be reconstructed. In very few instances of closely similar non-polyploid plant species is there evidence to suggest that one is the ancestor of another. A notable example is the cytological evidence for the origin of Crepis fuliginosa (n-= 3) from C. neglecta (n = 4) or its near ancestor (Tobgy, 1943). Similarly, Sherman (1946) showed that Crepis kotschyana (n = 4) has been derived from an ancestor like C. foetida (n 5). Another convincing example of an aneuploid species with a living diploid ancestor was described by Lewis and Roberts (1956) in their studies of Clarkia biloba (n= 8) and C. lingulata (n -9). These two species are so similar that they can only be distinguished by petal shape and chromosome number. Cytogenetic analysis showed that C. lingulata has an additional chromosome composed of parts of two chromosomes of the C. biloba genome. The presence of duplicated chromosome material in C. lingulata clearly establishes it as a derivative of C. biloba. Recently, Jackson (1962) described a case in Haplopappus that also suggests the origin of one diploid species from another. What was considered Haplopappus gracilis was found to consist of two extremely similar species, one with a chromosome number of n = 4, the other with n = 2. Cytological evidence indicates in this instance, as in Crepis, that aneuploid reduction is the most probable explanation for the difference in chromosome number, with the n = 2 species the derived one. The purpose of this paper is to present a unique example in Chaenactis (Compositae) in which the cytogenetical evidence indicates beyond reasonable doubt that an extant species of relatively mesic habitats, C. glabriuscula DC., has given rise independently to two similar desert species, C. fremontii Gray, and C. stevioides H. & A., by aneuploid reduction in chromosome number. Chaenactis comprises approximately 24 herbaceous species, all endemic to western North America. There are at least nine annual species, and four of these, C. glabriuscula, C. stevioides, C. fremontii, and C. xantiana form a closely related assemblage,2 judging from their similarity and frequent natural hybridization. The first three will be considered in detail in this paper. The yellow-flowered C. glabriuscula is the most variable, and some of its intergrading variants have been treated as separate species.3 Chaenactis stevioides and C. fremontii have white flowers, but are otherwise very similar morphologically to certain populations of C. glabriuscula. Despite their overall similarity, the gametic chromosome number of C. glabrius-