Comparitive Molecular Evolution Of 11S Globulin And Key Starch Synthesis Gene Families In Monocots And Dicots

Dicot seeds are generally rich in protein, which accounts for 40% of the dry soybean seed weight; and monocot seeds are generally rich in starch. These feathers are consequent of natural adaptation. As a result, the genes responsible for these feathers are changing accordingly. However, few studies have sought to understand how these genes changed and evolved. As more and more genomes of species were sequenced, there is a chance to shed some light on this question.To analyze the evolutionary pattern of key genes families for the globulin and starch synthesis and explore why monocots are rich in starch and dicots are rich in protein, in this study the molecular evolution of three kinds of gene families, being the 11S glycinin and 7Sβ-conglycinin gene family in soybean, the 11S globulin gene family in monocots and dicots, and the key starch synthesis gene families in monocots and dicots, were studied. The results were as follows.First,5 dicots (A. thaliana, G. max, M. truncatula, P. trichocarpa and V. vinifera) were used as material to investigate molecular evolutionary mechanism of 11S glycinin and 7Sβ-conglycinin gene families in soybean, by analyzing gene duplication, evolutionary rate, positive selection and divergence of the two gene families. The ancestral glycinin gene initially experienced a tandem duplication event; then, the genome underwent two subsequent rounds of whole-genome duplication, thereby resulting in duplication of the glycinin genes, and finally a tandem duplication likely gave rise to the Gyl and Gy2 genes. Theβ-conglycinin genes primarily originated through the more recent whole-genome duplication and several tandem duplications. Purifying selection has had a key role in the maintenance of genes in both gene families. In addition, positive selection in the glycinin genes and a large deletion inαβ-conglycinin exon contribute to the diversity of the duplicate genes. This means that the duplicated genes in both gene families prefer to retain similar function throughout evolution and therefore may contribute to phenotypic robustness.Second,5 monocots (O. sativa, B. distachyon, S. italica, Z. mays and S. bicolor) and 5 dicots (A. thaliana, G. max, C. sativus, P. trichocarpa and R. communis) were used as material to investigate the relationship of higher protein content in dicots with evolutionary mechanism of the 11S globulin gene family by analyzing gene duplication, evolutionary rate and positive selection of the gene family in monocots and dicots. The results showed more gene duplications of the 11S globulin gene family in the five dicots, O. sativa and B. distachyon, which are rich in 11S globulins; a higher evolutionary rate in the five dicots; and positive selections in the five dicots but not in the five monocots. These results provide the evidence for the guess that gene duplication and accelerated evolutionary rate may contribute to the higher ability of dicots to produce protein.Finally,5 monocots (O. sativa, B. distachyon, S. italica, Z. mays and S. bicolor) and 5 dicots (A. thaliana, G. max, M. truncatula, P. trichocarpa and R. communis) were used as material to investigate whether the gene duplication and accelerated evolutionary rate contribute to the higher ability of monocots to produce starch, by comparing the differences in gene duplication, evolutionary rate and positive selection of key starch synthesis gene families, i.e. AGPase, starch synthase, branching enzyme and debranching enzyme, between monocots and dicots. The results showed (ⅰ) distinct differences in the patterns of gene duplication and loss between monocots and dicots; duplication mainly occurred prior to the divergence of monocots, whereas duplication mostly occurred in individual species within the dicots; there is less gene loss in monocots than in dicots; (ⅱ) a considerably higher evolutionary rate, from 1.29 to 3.37 times, in monocots than in dicots in most gene families analyzed; (ⅲ) evidence of different selective pattern between monocots and dicots; positive selection may have occurred asymmetrically in monocots in some gene families, e.g. AGPase small subunit. Considering the distinct different in seed starch content between the two types of species, we therefore hypothesized that gene duplication and a higher evolutionary rate are probably the explanation for why monocots can produce more starch than dicots.These results above not only suggest that special evolutionary pattern (e.g., gene duplication and accelerated evolutionary rate) contributes to special phenotype (e.g., dicot seeds are rich in protein and monocot seed are rich in starch), but also shed light on how evolutionary divergence contributes to differences in morphological and physiological features between monocots and dicots.