Production of fertile hybrid germplasm with diploid Australian Gossypium species for cotton improvement

The 17 wild Australian Gossypium species are distant diploid relatives of t he commercial tetraploid cottons, G. barbadense L. and G. hirsutum L. They interest cotton breeders as a source of terpenoid-aldehyde-free seeds, a tr ait only found in five Australian Gossypium species. They elicit further in terest because some species grow near current and projected cotton growing areas in Australia and thus could serve as unintentional recipients of tran sgenes from genetically engineered cotton cultivars. The utility of the wil d Australian Gossypium species in cotton breeding depends on the ability to generate fertile hybrids, and to the extent this is possible under glassho use conditions, it allows predictions regarding the probability that fertil e hybrids between the transgenic cottons and spatially associated populatio ns of wild species will arise without human manipulation. The Australian Go ssypium species fall into three morphologically and cytologically distinct groups designated the C, G, and K genomes, The G-genome species hybridize m ost readily with G. arboreum (a diploid A-genome cultivated cotton), while the C- and K-genome species are more compatible with G. hirsutum (a tetrapl oid AD-genome cultivated cotton). These intergenomic hybrids are sterile, a nd the chromosome complement of the hybrids must be doubled prior to backcr ossing to G. hirsutum. The only exceptions were four G. hirsutum x K-genome triploids, which exhibited limited female fertility when backcrossed to G. hirsutum. Two of the three diploid species geographically associated with commercial cotton fields (G. australe F. Mueller & G. rotundifolium Fryxell , Craven & Stewart) failed to produce hybrid progeny when pollinated with G . hirsutum pollen; the third species (G. sturtianum J.H. Willis) produced o nly 5 sterile triploids from 25 pollinations. Thus, the probability that wi ld species could serve as recipients of transgenes is functionally zero, es pecially in conjunction with the profound prezygotic barriers that separate the cultivated tetraploid cottons from their wild Australian relatives. Ei ghteen new fertile synthetic polyploids and 23 self-fertile derivatives of two synthetic hexaploids were produced. Synthetic tetraploids require great er effort to backcross than do synthetic hexaploids. These fertile hybrids represent a new avenue of introgression of genes from wild Australian Gossy pium species into commercial cotton cultivars, an avenue limited only by th e level of recombination.