New World tetraploid cottons contain Old World cytoplasm

Abstract

New World tetraploid cottons (Gossypium spp.) originated through hybridization of ancestral diploid species that presently have allopatric ranges in Asia-Africa (the A genome) and the New World tropics and subtropics (the D genome). Despite intensive study, the identity of the parental diploids and the antiquity of polyploidization remain unresolved. In this study, variation in the maternally inherited chloroplast genome was assessed among species representing both of the parental genomes and the tetraploids. Approximately 560 restriction sites were assayed in each accession, representing sequence information for about 3200 nucleotides. The resulting maternal phylogeny has no convergent restriction site mutations and demonstrates that the cytoplasm donor for all tetraploid species was an A genome diploid with a chloroplast genome that is similar to Gossypium arboreum and Gossypium herbaceum. No mutational differences were detected between these two species, and few mutations distinguish the chloroplast genomes of A genome diploids from those of tetraploid taxa. In contrast to expectations based on extensive taxonomic, geographic, and genetic diversity, a surprisingly low level of sequence divergence has accumulated subsequent to polyploidization. Chloroplast genomes of tetraploid species are distinguished from each other by between one and six apparent point mutations. The data suggest that tetraploid cotton originated relatively recently, perhaps within the last 1-2 million years, with subsequent rapid evolution and diversification throughout the New World tropics.The cultivated cottons and their wild relatives (Gossypium spp.) have played an important role in the history of plant cytogenetics and speciation theory. Current classifications (1-3) recognize approximately 42 species distributed throughout the warm arid regions of the world, comprising either 5 or 6 tetraploid taxa and about 36 diploid species divided into 7 cytogenetic groups or "genomes" (4). Included in the genus are the morphologically diverse and geographically widespread cultivated cottons, consisting ofthe AsianAfrican diploids Gossypium arboreum L. and Gossypium herbaceum L. (2n = 26) and the New World tetraploids Gossypium barbadense L. and Gossypium hirsutum L. (2n = 4x = 52). Pioneering cytogenetic investigations by Beasley

Pros and cons of using genomic SSRs and EST-SSRs for resolving phylogeny of the genus Gossypium

“…Such commonalities have been found using RAPD markers (Khan et al 2000), cpDNA, ITS and combined data set based phylogenies (Seelanan et al 1997) and cpDNA restriction site based phylogeny ). The sister clustering of A-genome species with the AD-genome species strengthens the concept that A-genome is the cytoplasmic donor of AD-genome (Wendel 1989). It is likely that genomes of A-genome species have chromosome of larger size and more recurrence of repetitive sequences in their genome as compared to D-genome (Geever 1980), thus producing opportunities to amplify similar sequences (homology) among the genomes.…”

“…lanceolatum (AD)'. In earlier reports, high genetic similarity of G. longicalyx was reported with A-genome or allotetraploid derivatives (Wendel 1989). However, in majority of the earlier reports, relatively isolated position of G. longicalyx was reported (Phillips and Strickland 1966;Saunders 1961;Wu et al 2007).…”

“…It has also been observed that G. raimondii (D 5 ) and G. gossypioides (D 5 ) are genetically more comparable to G. hirsutum (AD 2 ) and G. barbadense (AD 2 ) (Kebede et al 2007). In few cytogenetic studies, it was elucidated that G. herbaceum is more comparable to the ancestor species of tetraploid cotton than G. arboreum (Endrizzi et al 1985;Wendel 1989;Percival and Kohel 1990).…”

Polyploid formation in cotton is not accompanied by rapid genomic changes

“…AADD; 2n = 4x = 52). These species originated following hybridization between an African or Asian diploid species (genome AA; 2n = 26), as female Wendel 1989), with a diploid American pollen donor (genome DD; 2n = 26). Molecular data suggest that the allopolyploid Gossypium lineage arose about 1-2 million years ago, with divergence of the two progenitor diploid genomes occurring 4-8 million years earlier (Seelanan et al 1997;Wendel and Albert 1992).…”

Genetic Mapping in Cotton

“…Tetraploid species evolved ~1-2 million years ago (MYA) as a result of hybridization between "A" and "D" genome species [16], diverged each other from a common ancestor about 4-11 (MYA) [6,17]. Both "A" & "D" genomes have maintained some level of sequence similarity, resulting in a high transferability of markers among the Gossypium species [18,19].…”