Relationships And Autopolyploid Evolution In The Medicago Sativa Complex (Alfalfa And Allies; Leguminosae)

The Medicago sativa 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 evolutionary relationships among taxa in the complex, sequence variation in two noncoding regions of cpDNA (rpl20-rps12 and trnS-trnG spacers) and three regions of mitochondrial DNA (mtDNA: nad4 intron, nad7 intron, and rpS14-cob spacer) were surveyed from 48 (37 for mtDNA) individuals representing these and other diploid taxa in the complex. These sequences afforded independent perspectives on the evolutionary history of the group, because mtDNA is maternally inherited in Medicago whereas cpDNA is biparentally inherited with strong paternal bias. Twenty and 21 haplotypes were identified for cpDNA and mtDNA, respectively. Haplotype networks were constructed and tests of differentiation were conducted. Results from cpDNA sequences supported the recognition of “caerulea” and “falcata” as differentiated taxa, despite the presence of some shared haplotypes, in agreement with morphological characters. In contrast, no significant evidence of mtDNA haplotype differentiation was observed. Incongruence between cpDNA and mtDNA is more likely explained by introgression of the mitochondrial * This chapter was published in Havananda, T. , E. C. Brummer, I. J. Maureira-Butler, and J. J. Doyle. 2010. Relationships among diploid members of the Medicago sativa (Fabaceae) species complex based on chloroplast and mitochondrial DNA sequences. Systematic Botany 35: 140–150. Written authorization for the use of the material in this dissertation was obtained from Systematic Botany. 2 genome than by incomplete lineage sorting of mtDNA haplotypes, given the expected smaller effective population size for uniparentally inherited mtDNA than for biparentally inherited cpDNA. Moreover, the two taxa are readily crossable, making natural hybridization possible. The long-time disagreement on whether to recognize “falcata” as a separate species or a subspecies of M. sativa s. l. is due to the common problem of unequal rates of differentiation for different characters during speciation. Introduction Polyploidy is one of the most important forces driving plant evolution and diversity; it can give rise to genomic rearrangements, including exchanges between genomes and gene loss, and changes in gene expression (e.g., Wang et al., 2006; Gaeta et al., 2007). Many recent studies have focused on various aspects of polyploidy (Soltis et al., 2004b) to understand the process of polyploidization (e.g., reviewed in Wendel, 2000) and its consequences for plant evolution (e.g., Soltis and Soltis, 2000; Adams and Wendel, 2005; Comai, 2005; Adams, 2007). However, more studies have been done in allopolyploids than in autopolyploids (e.g., Doyle et al., 2004; Soltis et al., 2004a; Dubcovsky and Dvorak, 2007). Genetic studies have revealed that autopolyploidy is much more common than generally recognized, yet autopolyploidy has often been overlooked as an important part of evolution in natural populations (Soltis et al., 2004b; Soltis et al., 2007). The Medicago sativa L. polyploid complex contains diploid and autotetraploid cytotypes and has potential to be a good model system for evolutionary study of autopolyploidy. Because alfalfa (M. sativa), an autotetraploid (Stanford, 1951; Quiros, 1982) member of the complex, is one of the world’s most important forage crops, a considerable amount of genetic information and resources such as germplasm collections exist and are available for the study of this complex. In