1 College of Grassland Science and Technology, Sichuan Agricultural University, Wenjiang 611100, Chengdu China; pengyanlee@163.com
2 Guangdong Technology Research Center for Marine Algal Bioengineering, Guangdong Key, Laboratory of Plant Epigenetics, College of Life Sciences and Oceanography, Shenzhen University, Shenzhen 518055, China; anwar_uaar@yahoo.com
* Correspondence: m.zafarsindhu@hotmail.com (M.Z.I.)
Abstract: Interspecific hybridization has contributed significantly to land diversity, species evolution, and crops’ domestication, including upland cotton, the cultivated form ofGossypium hirsutum . Being the world’s most important fiber crop species, Gossypium hirsutum belongs to the allotetraploid Gossypium consisting of six additional tetraploid species. The lint fiber evolved once in diploid parent A-genome species in theGossypium ’s history and passed on during hybridization of the A-genome with the D-genome and was maintained in subsequent evolution. The domestication history of G. hirsutum involved the collection and use of lint fibers by indigenous people for the purpose of making strings and other textile products; hence, spinnable lint fibers were likely to have evolved under domestication. Crossing with G. barbadense has resulted in the development of multiple genetic lines in contemporary upland cotton. However, in later-generation hybrids betweenG. hirsutum and other polyploid species, reproductive barriers such as reduced fertility, segregation distortion, and hybrid breakdown are frequently observed, complicating the task of introgressing new, stably inherited allelic variation from inter-specific hybridization. Recent efforts in molecular genetics research have provided insights into the location and effects of QTLs from wild species that are associated with traits important to cotton production. These and future research efforts will undoubtedly provide the tools that can be utilized by plant breeders to access novel genes from wild and domesticated allotetraploid Gossypium for upland cotton improvement.
Keywords: interspecific hybridization; Gossypium hirsutum; crop improvement; fiber quality; polyploidization; introgressive breeding
1. Introduction
Cotton is a food and fiber plant contributing to humanity’s fundamental requirements. Cotton fiber, in the form of textile objects, contributes significantly to the comfort, style, and culture of human society. Despite its lack of appeal as a food, cotton is a primary source of vegetable oil, which is used extensively in meals such as baking and frying fats, mayonnaise, margarine, and snack food. Following oil extraction, the seed by-product is used as a raw material in animal feed, fertilizer, and paper. Because of its flexibility, cotton is one of the most important field crops in the world. According to the International Cotton Advisory Committee (ICAC), which collects data on global cotton production, consumption, and commerce, cotton is grown on 36 million hectares in over 100 countries [1]. The top five cotton-producing countries are China, India, the United States, Pakistan, and Brazil, which account for around two-thirds of worldwide cotton production [2]. Cotton has become the most important natural fiber in the textile industry, as well as a vital agricultural product in the global economy. The annual value of the worldwide cotton crop is estimated to be over USD 30 billion, with lint fiber accounting for 90% of that value [3]. Cotton farming and processing employ about 350 million people worldwide. Cotton’s economic importance as a natural fiber for the global textile industry has spurred a lot of interest in improving the crop’s inherent genetic potential by breeding cultivars with higher biotic and abiotic tolerance, higher lint yield, and enhanced fiber quality [4].
Cotton’s development and progress have benefited greatly from interspecific hybridization. Cotton remarkable as a crop in that four distinct species of the genus Gossypium (Malvaceae) were domesticated for lint fiber production on two separate continents [5,6]. As a result, in the textile business, the two allotetraploid speciesG. hirsutum L. and G. barbadense L., both native to the Americas, and the two diploid species G. arboreum L. and G. herbaceum L., both endemic to Africa and Asia, are referred to as “cotton.” Allotetraploid Gossypium has a single polyploidization history, with progenitors that resemble G. herbaceum or G. arboreum (A genome) and G. raimondii (D genome) merging together [6]. The two allotetraploids now provide the great majority of the world’s textile fiber, with G. hirsutumaccounting for more than 90% of worldwide cotton production [7]. As a result, the focus of this review will be on G. hirsutum , popularly known as “Upland” cotton, a cultivated allotetraploid species. Other domesticated and non-domesticated allotetraploid species, on the other hand, will be included since they represent a vast pool of untapped genetic resources for future genetic advancement.
2. Taxonomy of Gossypium and Origin of Gossypium hirsutum
The genus Gossypium contains around 50 species, which are distributed in tropics and subtropics regions. Diploid Gossypium species are categorized into eight genome groups (A–G, K) based on chromosomal pairing affinities [8]. Despite the fact that numerous important diversification sites have been identified, these species collectively have a large distribution. A-genome species are found both in Africa and Asia; B- and F-genome species are mainly present in Africa; Arabia has E-genome species; C-, G-, and K-genome species can be found in Australia; whereas D-genome species can be found in Central and South America. The detailed grouping and geographical distribution of Gossypium species are summarized in Table 1. Although molecular phylogenetic investigations have established a phylogenetic framework for the genus and its many genome types, a thorough understanding of the evolutionary links between each species remains elusive [6]. G. hirsutum and G. barbadense , two domesticated species, plus G. tomentosum Nutt ex Seem, G. darwinii Watt, and G. mustelinum Miers ex G. Watt, are the five allotetraploid Gossypium species that have been extensively recognized in the past. G. ekmanianum Wittmack, a sixth species, was recently discovered, and a seventh was discovered (see below). Polyploid Gossypium has a broad geographic distribution that includes several seasonally dry subtropical and tropical parts of the North and South American continents, mainly near coasts, as well as numerous Caribbean and Pacific islands. As a result, these species are commonly referred to as New World cotton. The fact that G. darwinii (Galapagos Islands), G. ekmanianum (Hispaniola), andG. tomentosum (Hawaiian Islands) are all island endemics suggests that these species evolved after long-distance dispersion episodes [6,9]. The native Brazilian cotton (Gossypium mustelinum) has never been cultivated. It is tetraploid and may be crossed with cultivated cotton. Gossypium mustelinum is grown in semiarid region and expanded to the coastal area of northeast Brazil in 2018. It is cultivated in Paraiba and Pernambuco in Brazil [10]. G. hirsutum is native to Central America, whereas G. barbadense is native to South America, but their ranges overlap, notably in northwest South America and across the Caribbean. Two new species have just been added to the allopolyploid cotton group. Based on accessions obtained in the Dominican Republic, the species G. ekmanianum Wittm was recently revived [11,12]. A seventh species, Gossypium stephensii[13], has been recently discovered in the Wake Atoll in the Pacific Ocean (Wake, Peale, and Wilkes Islands) [13,14].G. hirsutum , G. tomentosum , G. ekmanianum, and the newly found species form one clade; G. barbadense and G. darwinii form a second clade; and G. mustelinum remains the allopolyploid phylogeny’s basal clade [6]. Because of their monophyletic origin, all seven polyploid species have two sets of 13 homoeologous chromosomes (2n = 4x = 52) and strict disomic chromosome pairing (Kimber 1961) A-genome cytoplasm. Polyploidization occurred about 1–2 million years ago by transoceanic migration of an Old World (A-genome) progenitor followed by hybridization with a native New World (D-genome) progenitor [6]. The At- and Dt-subgenomes of the tetraploids were provided by diploid progenitors that resemble G. arboreum or G. herbaceum , and G. raimondii , respectively, according to meiotic chromosomal pairing and comparative genome studies. The progenitors of these species separated from a common ancestor 5–7 million years ago, according to DNA sequence evidence [6,15]. Cotton’s A- and D-genome progenitors have at least nine chromosomal rearrangements in common [16]. Furthermore, the A-genome contains around twice as much gametic DNA as the D-genome [17], with the increased genome size attributed mostly to the repetitive DNA fraction, as the quantity of single-copy DNA in both genomes is nearly equal [18]. Genetic mapping [19,20] identified the matching homoeologous chromosomes of the At- and Dt-subgenomes, which were recently confirmed by two draft genomes sequences [14]. In the At sub genome, direct comparisons of gene order and synteny between the two sub genomes revealed two reciprocal translocations between chromosomes 02/03 and 04/05, as well as multiple probable inversions [18,19]. The tetraploid chromosomes have also been aligned with those of their diploid progenitors, indicating that there has been more rearrangement since polyploid development [20,21]. Nonetheless, these findings suggest that gene collinearity between the two subgenomes is substantial, and that chromosomal structural rearrangement has been minimal after allopolyploid formation. A grouping of Gossypiumspecies based on genomic diversity is shown in Table 1, and the evolution of Gossypium species, including tetraploidGossypium species formation, is illustrated in Figure 1.
Table 1. Grouping and geographical distribution of the genusGossypium species based on genomic diversity validated by cytogenetic data and other means.