Evolutionary Conservation and Divergence of Genes Encoding 3-Hydroxy-3-methylglutaryl Coenzyme A Synthase in the Allotetraploid Cotton Species Gossypium hirsutum

Polyploidization is important for the speciation and subsequent evolution of many plant species. Analyses of the duplicated genes produced via polyploidization events may clarify the origin and evolution of gene families. During terpene biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMGS) functions as a key enzyme in the mevalonate pathway. In this study, we first identified a total of 53 HMGS genes in 23 land plant species, while no HMGS genes were detected in three green algae species. The phylogenetic analysis suggested that plant HMGS genes may have originated from a common ancestral gene before clustering in different branches during the divergence of plant lineages. Then, we detected six HMGS genes in the allotetraploid cotton species (Gossypium hirsutum), which was twice that of the two diploid cotton species (Gossypium raimondii and Gossypium arboreum). The comparison of gene structures and phylogenetic analysis of HMGS genes revealed conserved evolution during polyploidization in Gossypium. Moreover, the expression patterns indicated that six GhHMGS genes were expressed in all tested tissues, with most genes considerably expressed in the roots, and they were responsive to various phytohormone treatments and abiotic stresses. The sequence and expression divergence of duplicated genes in G. hirsutum implied the sub-functionalization of GhHMGS1A and GhHMGS1D as well as GhHMGS3A and GhHMGS3D, whereas it implied the pseudogenization of GhHMGS2A and GhHMGS2D. Collectively, our study unraveled the evolutionary history of HMGS genes in green plants and from diploid to allotetraploid in cotton and illustrated the different evolutionary fates of duplicated HMGS genes resulting from polyploidization.