Abstract
The wild cotton diploid species (2n = 2x = 26) are important sources of useful traits such as high fiber quality, resistance to biotic and abiotic stresses etc., which can be introgressed into the cultivated tetraploid cotton Gossypium hirsutum L (2n = 4x = 52), for its genetic improvement. The African wild diploid species G. longicalyx Hutchinson and Lee could be used as donor of the desirable traits of fiber fineness and resistance to reniform nematode. However, hybridization of wild diploid species and cultivated tetraploid cotton encounters a sterility problem of the triploid (2n = 3x = 59), mainly due to ploidy. The restoration of the fertility can be done by creating an allohexaploid (2n = 6x = 78) through the doubling with colchicine of the sterile triploid chromosomes. With this method, a synthetic allohexaploid hybrid (G. hirsutum × G. longicalyx)2 has been obtained. This genotype was studied using phenotypic, cytological and molecular (AFLP) analyses in order to confirm its hybridity and its caryotype, and also to verify the expression of the desirable traits coming from G. longicalyx. The studied genotype showed a quite good level of pollen fertility (83%), and apart from larger seeds and some minor seedling anomalies, most of its morphological characteristics were intermediate between the two parental species. It had 78 chromosomes, proving its hexaploid status. Molecular analysis revealed 136 AFLP loci in this hexaploid, all from G. hirsutum and G. longicalyx, demonstrating its hybrid status. In addition, the hexaploid exhibited the useful traits of G. longicalyx with regard to its remarkable fiber fineness and its high resistance to the reniform nematode. This allohexaploid hybrid constitutes an interesting agronomic material, which can be used as a bridge for the transfer of useful agronomic traits from wild species to varieties of G. hirsutum.
Highlights
Cotton, from the genus Gossypium, is the most important natural fiber source for the textile industry in the world [1]
The wild cotton diploid species (2n = 2x = 26) are important sources of useful traits such as high fiber quality, resistance to biotic and abiotic stresses etc., which can be introgressed into the cultivated tetraploid cotton Gossypium hirsutum L (2n = 4x = 52), for its genetic improvement
Analysis of the mitotic metaphase plates showed 52 chromosomes for G. hirsutum, 26 chromosomes for G. longicalyx and 78 chromosomes for the putative (G. hirsutum × G. longicalyx)2 hexaploid hybrid (Figure 1). This number of 78 chromosomes proves the hexaploid status of the material studied because it is in agreement with the number of chromosomes expected for a synthetic hexaploid cotton (2n = 6x = 78) resulting from the doubling of the chromosomes of a triploid issued from the cross between a tetraploid cotton (G. hirsutum, 2n = 4x = 52) and a diploid cotton (G. longicalyx, 2n = 2x = 26)
Summary
From the genus Gossypium, is the most important natural fiber source for the textile industry in the world [1]. The Gossypium genus is composed of 53 species among which 7 are tetraploid (2n = 4× = 52) and 46 are diploid (2n = 2x = 26) [2]. Two cotton diploids species (G. arboreumn L and G. herbaceum L) and two tetraploids species (G. hirsutum L and G. barbadense L) are cultivated for their spinnable fiber [6] [7] [8]. The remaining cotton fiber supply is produced from the other three cultivated cottons [9]. Apart from these four species, the other 46 species of Gossypium are wild
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