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

Abstract

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, nad 7 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 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.

Complex patterns of autopolyploid evolution in alfalfa and allies (Medicago sativa; Leguminosae)

“…Medicago sativa is relatively unusual among flowering plants in having predominantly paternal transmission of cpDNA (Schumann and Hancock, 1989; Smith, 1989; Masoud et al, 1990). Here we follow up our finding of chloroplast haplotype differentiation among the blue‐ and yellow‐flowered diploid members of this species (Havananda et al, 2010). We explore genetic diversity among tetraploid members of M. sativa using the same cpDNA regions to elucidate origins of tetraploids and to understand patterns of gene flow within and between ploidal levels.…”

“…However, much less is known about other tetraploids in this species or about the diploids hypothesized to have given rise to them and to alfalfa, despite the use of some of these taxa in alfalfa breeding programs. Indeed, it is still an open question whether there may be taxonomic allopolyploids in this species, given the morphological diversity and controversial taxonomic history of diploid members (e.g., Havananda et al, 2010).…”

“…Hypothesized relationships among taxa in Medicago sativa (Lesins and Lesins, 1979; Quiros and Bauchan, 1988; Small and Jomphe, 1989), modified from Havananda et al (2010). Arrows with solid lines denote autopolyploidy, and those with dashed lines denote hybridization.…”

“…Although there have been a number of studies using various kinds of molecular data to evaluate genetic variation among members in this species, most have focused on cultivated alfalfa (sativa) and included few accessions from other taxa (e.g., Kidwell et al, 1994; Crochemore et al, 1996; Diwan et al, 1997; Segovia‐Lerma et al, 2003), and few have addressed relationships involving its wild members (Brummer et al, 1991; Skinner, 2000; Muller et al, 2003, 2006). Most recently, the relationships of a wide range of wild diploid members of M. sativa were investigated by Havananda et al (2010), using cpDNA and mitochondrial DNA (mtDNA), and by Şakiroğlu et al (2010), using simple sequence repeat (SSR) markers, to estimate genetic diversity and infer population structure. Both of these studies showed that blue‐flowered and yellow‐flowered diploids in this species (caerulea and diploid falcata) are genetically differentiated, though this had not been detected in some previous studies (e.g., Skinner, 2000; Muller et al, 2006).…”

Applied Genetics and Genomics in Alfalfa Breeding

“…An extensive examination of wild diploid germplasm using simple sequence repeat (SSR) markers showed that falcata and caerulea accessions are clearly distinct, with hybrids falling in between the separate parental groups [10,11]. Falcata and sativa are similarly distinct, based on numerous experiments conducted on tetraploid populations [9,12,13].…”