**3. Genetic resource, origin, distribution and uses**

Genetic variability is the main driving force of all breeding programs. The basic requirement of varietal improvement/hybrid development/marker-assisted selection is the availability of genetic resources. Even basic molecular understanding requires the occurrence of special important morphological/physiological characters upon which the studies are imposed. It is therefore necessary to understand and utilize within and between species diversity for crop improvement. The Cotton, belonging to family Malvaceae and genus Gossypium has high species diversity, which includes diploids and tetraploids with all the diploids sharing a common chromosome number i.e. 2n = 2x = 26. However, with 3-fold variation in DNA content per genome [10], they are classified into eight cytological groups (A, B, C, D, E, F, G, and H) [11–13]. All the tetraploids also share the same chromosome number i.e. 2n = 4x = 52 and have an AD genome. So, in total, the family Gossypium has around 51 recognized species, which includes 7 tetraploids and 44 diploids (**Table 1**). The genome A is thought to be originated in Asia/Africa, but the D genome is a derivative of A-genome formed by allopatric speciation i.e. due to transoceanic dispersion (Africa to Peru) of the A-genome. The modern-day tetraploids (AD-Genome) have originated from the trans-oceanic dispersion of A-genome to Peru followed by a polyploidization event with the native D-genome of Peru [14]. Cotton fiber is a single cell extension of the seed cell epidermis with deposition of cellulose. Only four species underwent the parallel selection pressure of domestication in America (*Gossypium hirsutum* and *G. barbadense*; tetraploids) and Africa-Asia (*G. arboreum* and *G. herbaceum*; diploids) and only these species produce the seed epidermal cell extension that is between 10 mm to 35 mm and hence are





**Table 1.**

*Species diversity in cotton and their importance.*

cultivated for lint purpose. The rest of the species produce lint less than 10 mm with varying shades of brown to white. Some of the recent updates in number of species in the Gossypium family include *G. trifurcatum* being tentatively placed in the B genome [15]. *G. lanceolatum* was proved to be a domesticated form of *G. hirsutum* and it does not hold a species status [16]. *G. stephensii* and *G. ekmanianum* are the two new tetraploids discovered with a species status [17, 18]. Wild species are considered as the treasure of important genes required to combat biotic and abiotic stress [19]. The various species of the Gossypium genus and their important traits of interest are presented in **Table 1**.

*G.arboreum* and *G. herbaceum* species known as old-world cotton were majorly grown in the Indian sub-continent. The Indus valley discoveries prove that cotton was grown as early as in 6 millennium BC, with the use of cotton being mentioned in Rig Veda (15 century BC) and Manu's Dharmashastra (800 BC) [14]. From the Indian sub-continent cotton has spread to Mesopotamia, Egypt and Nubia. During the first century, cotton was introduced to Europe by the Arab traders. The East India Company that colonized India (1757) and started ruling were the biggest importers of raw cotton and they used to sell the finished goods to India and the world. *G. arboreum var neglecta* grown in Bengal was known to produce lint that could be spun to 480 counts yarn and made into Muslins which was a result of both a beautiful skill set and the cotton germplasm that were available. Garments produced here were called "webs of woven wind" [20]. However the East India Company that wanted to sell only their finished cotton garments, chopped off the thumbs of weavers and with the weavers lost, the germplasm too vanished from the world forever [21]. Though the polyploidization event of tetraploids happened in Peru, the *Gossypium hirsutum* and *G. barbadense* also called new world cottons originated in Mexico and Peru, respectively, from where it spread to both South and North America. The arrival of European colonists hastened the spread of the new world cotton to the rest of the world [22]. The East Indian Company also bought and introduced early maturing and high yield *G. hirsutum* cotton to India with many unsuccessful attempts being made (1790 in Bombay & Madras, 1840–42 in Deccan, Konkan and Hubli, 1853 in Punjab). However, the most significant development in terms of the spread of cotton was the introduction of Cambodia variety in Tamil Nadu region [23]. At present, *G. hirsutum* is cultivated in 95% cotton area due to its high yield ability. *G. barbadense* is the best source for fiber quality improvement of *G. hirsutum* as *G. barbadense* is known to produce lint


#### **Table 2.**

*Country-wise list of germplasm maintained.*

that can be spun to 80–120 counts yarn [24]. However, owing to their low yields, *G. arboreum*, *G. herbaceum,* and *G. barbadense* are not grown widely. Maintaining germplasm and utilizing within-species variation are big challenges in varietal development as it will be an expensive proposition. The germplasm maintained elsewhere in different countries can be efficiently utilized in breeding programs. The list of germplasm preserved is mentioned in **Table 2**. Commendable efforts are needed in pre-breeding to utilize the rare alleles/genes present in wild species. In the principal crops like rice and wheat, the IRRI and CIMMYT, respectively, are taking up the pre-breeding work and the genetic materials are being supplied to breeders around the world. Some notable works concerning cotton pre-breeding include developing populations involving genes/segments/whole chromosomes from wild species. RHMBHMTUP-C4 a random mated population was developed involving *G. hirsutum*, *G. barbadense*, *G. mustelinum* and *G. tomentosum* [31]. RMBUP-C4 was developed from crossing three elite hirsutum lines with 18 chromosome substitution lines from *G. barbadense* [32]*.* There is huge scope for pre-breeding work in cotton to combat biotic and abiotic stresses.
