Chloroplast DNA variation in Coreopsis nuecensoides and C-nuecensis (Asteraceae), a presumed progenitor-derivative species pair

Coreopsis nuecensoides and C. nuecensis are narrowly distributed endemics o f southeastern Texas. While they overlap in range, they differ in chromosom e number, and F-1 hybrids exhibit strong sterility barriers. Previous morph ological, cytogenetic, and allozyme studies suggested that C. nuecensoides and C. nuecensis are very closely related members of a progenitor-derivativ e species pair. The two species differ substantially in terms of their leaf flavonoid chemistry, but the flavonoid data do not address the exact natur e of the relationship between them. We have further examined the relationsh ip between the species by analyzing genetic diversity within both species u sing chloroplast DNA (cpDNA) restriction site data. Sixteen restriction sit e changes were used to define thirteen distinct cpDNA haplotypes. The patte rn of relationships among haplotypes provides some support for the presumed ancestor-descendent relationship, but other interpretations are possible. Only one cpDNA haplotype was shared by the two species; of the remaining tw elve, seven were unique to one species and five to the other. This result i s consistent with results of flavonoid studies, in which each species exhib ited flavonoid compounds not seen in the other, but differs from the allozy me results, in which the vast majority of alleles were shared by both speci es and only the presumed progenitor exhibited unique alleles. Taken togethe r, the data support a very close relationship between the species, but are equivocal with regard to a progenitor-derivative relationship.