Selective fertilization in interspecific crosses of allotetraploid species of Gossypium

The genus Gossypium is composed of both diploid and allotetraploid species. The five allotetraploid species of Gossypium are sexually compatible, and only partial sexual barriers have been described. Natural hybrids among them do not occur or occur in situ with very low frequency in Brazil in the rare places where cultivated upland cotton (Gos- sypium hirsutum L.) occurs in a sympatric range with Gossypium barbadense L. or Gossypium mustelinum Miers. We eval- uated the presence of pollen competition as a prezygotic barrier in crosses between upland cotton and G. barbadense and G. mustelinum. We found that G. barbadense pollinated with a mixture of 50% upland cotton and 50% G. barbadense pol- len resulted in 17.4%-31.1% interspecific hybrids, depending upon the upland cotton genotype used as pollen donor. Mix- tures containing pollen from G. mustelinum and upland cotton, used to pollinate G. mustelinum in proportions of 25%:75%, 50%:50%, and 75%:25%, produced 61.3%, 22.5%, and 3.6% interspecific hybrids, respectively. These low rates of hybrid production demonstrate that pollen competition is present with G. mustelinum and G. barbadense and confirms this mecha- nism as a sexual barrier.