Cytological studies in cotton

VII. Summary

1. Of two interspecific hybrids between Asiatic and New World cottons studied, one had 2n = 39 and the other 2n = 52. For the latter, the inference is that a diploid egg from Asiatic cotton had functioned.

2. In a study of the somatic chromosomes of New World cotton it has been found that half of the chromosomes are small and the other half larger, the latter being comparable in size to the chromosomes of Asiatic cotton. The small chromosomes of New World cotton are of the same size as those in diploid wild species from North America. Species from the Old World and from Australia are all characterised by the larger size of their chromosomes.

3. In the first meiotic division it was seen that (1) at least 13 univalent chromosomes are present in both hybrids, and (2) the hybrid with 52 chromosomes shows the same chromosome conjugation as in a triploid Asiatic cotton, but with the addition of an extra set of 13 non-homologous chromosomes.

4. The conclusion was drawn that New World cottons are allopolyploid species. It is thought that these probably originated from a cross between two species ofGossypium both withn = 13 but possessing morphologically dissimilar and non-homologous sets of chromosomes. The inference is that one of the parental species was an Asiatic cotton or a very closely allied type, while the other was probably a New World species characterised by its smaller chromosomes.