MOLECULAR EVIDENCE FOR HOMOPLOID RETICULATE EVOLUTION AMONG AUSTRALIAN SPECIES OF GOSSYPIUM

Abstract

Abstract. -Interspecific hybridization and introgression are important evolutionary processes in plants, but their full significance with respect to speciation at the diploid level remains unresolved. In this study, molecular markers from the plastid and nuclear genomes were used to document an unusual evolutionary history of Gossypium bickii Prokh. (Malvaceae). This species is one of three morphologically similar Australian cottons (along with G. australe F. Muell. and G. nelsonii Fryx.) in section Hibiscoidea. In contrast to expectations based on previous morphological data, cladistic analysis of maternally inherited cpDNA restriction site mutations unites G. bickii with G. sturtianum J. H. Willis, a morphologically distant species in a different taxonomic section (Sturtia).Few restriction site mutations distinguish the plastomes of G. bickii and G. sturtianum, but these two cpDNAs are differentiated from those of G. australe and G. nelsonii by a minimum of 33 mutations (out of 640 sites scored). These two highly distinct clades are not supported by phylogenetic analyses ofallozyme markers (from 58 populations) and restriction site mutations in nuclear ribosomal DNAs. Rather, phylogenies based on 83 nuclear markers indicate that G. bickii shares a more recent common ancestor with G. australe and G. nelsonii than it does with G. sturtianum. We suggest that the striking discrepancy between independent molecular phylogenies from two different genomes indicates a biphyletic ancestry of G. bickii. Our preferred hypothesis involves an ancient hybridization, in which G. sturtianum, or a similar species, served as the maternal parent with a paternal donor from the lineage leading to G. australe and G. nelsonii. Because we detected no G. sturtianum nuclear genes in G. bickii, we suggest that the nuclear genomic contribution of the maternal parent was subsequently eliminated from the hybrid or its descendent maternal lineage. Several possible mechanisms of cytoplasm transfer are suggested, including repeated backcrossing of the hybrid, as female, into the paternal donor lineage, selection against recombinant nuclear genomes and a form of apomixis known as semigamy. This example, and several others in Gossypium as well as other genera, attest to the evolutionary possibility of interspecific cytoplasmic transfer, and perhaps the origin ofdiploid species via reticulate speciation. In addition, this study offers an example of natural cytoplasmic introgression without long-term survival of nuclear genes from the maternal progenitor.

Chloroplast DNA Structural Variation, Phylogeny, and Age of Divergence among Diploid Cotton Species

“…robinsonii (C 2 ) and G . populifolium (K), supporting the proposal [ 52 – 53 ] that G . bickii has an introgressive ancestry with a maternal donor from the G .…”

Chromosomal Locations of 5S and 45S rDNA in Gossypium Genus and Its Phylogenetic Implications Revealed by FISH

“…The numbers of 45S rDNA were revealed three and four in G 3 and G 1 , respectively, although the two G-genome species have similar morphological traits [47]. It is a certainty for G genome in terms of the taxonomy, which had been well studied [40,47].…”

“…It is a certainty for G genome in terms of the taxonomy, which had been well studied [40,47]. Therefore, the origin might account for the difference in the rDNA numbers between two species.…”

Cytonuclear Coevolution following Homoploid Hybrid Speciation inAegilops tauschii

“…Historical evidence of this process has emerged from genetic and genomic analyses of nuclear genes and from discordance between organellar and nuclear markers (Arnold et al. 1988; Rieseberg 1991; Wendel et al. 1991; Dowling and Secor 1997; Hermansen et al.…”