Relationships among Diploid Members of the Medicago sativa (Fabaceae) Species Complex Based on Chloroplast and Mitochondrial DNA Sequences

The Medicago satizu species complex includes tetraploid cultivated alfalfa and several other diploid and tetraploid taxa that are recognized either as subspecies of M. sativa or as separate species. The two principal diploid taxa are caerulea, with purple flowers and coiled pods, and falcata with yellow flowers and falcate pods. TO Understand the 0:01LItionary relationships among taxa in the complex, sequence variation in two noncoding regions of cpDNA (rp120-rps12 and trnS-trnG spacers) and three regions of mitochondrial DNA (mtDNA: nad4 intron, nad7 intron, and ipS14-cob spacei) were surveyed from 48 (37 for mtDNA) individuals representing these and other diploid taxa in the complex. These sequences afforded independent perspectives oil the evolutionary historv of the groop, becanse rntf)NA is illaternally inherited in AIctlicago whereas cpDNA is biparentally inherited with strong paternal bias. Twenty and 21 haplotypes were identified for cpl)NA and nitDNA, respectively. Haplotype networks were constructed and tests of differentiation were conducted. RESULTS frorn cpDNA seqUences supported the recognition of cacrulea and falcata as differentiated taxa, despite the presence of some shared haplotypes, in agreement with) niorphological characters. In contrast, no significant evidence of nitlDNA haplotype differentiation was observed. Incongruence between cpDNA and mtDNA is more likey explained by introgression of the rnitochondrial genorne than bv incomplete lineage sorting of nltlDNA haplotypes, given the expected smaller effective population size for uniparentally inherited nitDNA than for biparentally inherited cpDNA. Moreover, the two taxa are readily crossable, making natural hybrudizaion possible. The long-time disagreement oil whether to recognize falcata as a separate species 01 a subspecies of AT saliva s. I. is due to the common problem Of UneqUal rates of differentiation for different charactens during speciation.