Relationships And Autopolyploid Evolution In The Medicago Sativa Complex (Alfalfa And Allies; Leguminosae)
The Medicago sativa complex comprises several morphologically and genetically diverse diploid and autopolyploid taxa, including autotetraploid cultivated alfalfa (M. sativa subsp. sativa). Its members can be divided into three morphological groups: M. sativa subsp. caerulea and subsp. sativa with blue flowers and coiled pods, subsp. falcata with yellow flowers with falcate pods, and subsp. glomerata with yellow flowers and coiled pods; each group contains both diploid and tetraploid cytotypes. Although alfalfa is well studied, closely related tetraploids and their hypothesized diploid progenitors have received much less study. Questions regarding their relationships, their controversial taxonomy, and autopolyploid evolution remain to be addressed. Genetic variation and differentiation were estimated, and phylogenetic and network relationships were constructed based on nucleotide sequences from the mitochondrial genome for the diploid members of the complex, and from chloroplast and nuclear genomes for both the diploid and tetraploid members. Independent perspectives on the species' evolutionary history were afforded because each genome has a different inheritance pattern. Mitochondrial DNA is maternally inherited, chloroplast DNA is biparentally, but largely paternally inherited, and nuclear DNA is biparentally inherited. At the diploid level, subsp. caerulea is genetically differentiated from diploid subsp. falcata for chloroplast haplotypes and nuclear alleles, although there are some shared haplotypes and alleles probably due to limited gene flow. Data from mitochondrial haplotypes, however, show no differentiation between the two diploids, which is likely due to bidirectional introgression of the mitochondrial genome. At the tetraploid level, genetic differentiation was found between subsp. sativa and tetraploid subsp. falcata in both the chloroplast and nuclear genomes. Although chloroplast data support a simple autopolyploid origin of subsp. sativa from diploid subsp. caerulea, a contrasting history involving past introgression from closely related M. prostrata is suggested for tetraploid subsp. falcata, raising questions about its autopolyploid origin. Nuclear data, however, show that tetraploid falcata most likely has originated from diploid falcata through autopolyploidy in a similar pattern to that of subsp. sativa. Despite the existence of hybrids, gene flow and introgression are limited and morphologically and genetically distinctive subspecies persist.
alfalfa; autopolyploidy; network analysis
Doyle, Jeffrey J
Viands, Donald Rex; Luckow, Melissa A
Ph. D., Plant Breeding
Doctor of Philosophy
dissertation or thesis