Figure 1. The evolution of Gossypium genus and the
formation of tetraploid Gossypium species. MB show genome size of
representative species.
3. Domestication of Upland Cotton
After polyploidization, G. hirsutum evolved to produce
high-quality fiber and to best survive against adverse environments
[7]. The domestication history of G. hirsutum is similar to
that of the other three domesticated cotton species; indigenous peoples
may have gathered and employed lint fibers for string and other textile
items. [7,20,21]. Domestication of cotton gave rise to long lint
fiber, which has a flat convoluted ribbon shape that allows it to be
spun into yarn [20,22]. Short
‘linters’ or fuzz stick to the seed coat tightly, whilst longer ‘lint’
fibers cling to the seed coat loosely. Fuzz fibers are an important
source of raw material for paper and other industrial products. Cotton
is the world’s most important fiber crop due to its longer, spinnable
lint fibers, and these novel single-celled seed epidermal trichomes may
have lured ancient peoples to the cotton plant in the first place. Four
separate Gossypium species were domesticated independently by four
civilizations on two continents, as previously stated: A-genome diploidsG. herbaceum and G. arboreum were domesticated in Africa
and Asia, and allopolyploids G. hirsutum and G. barbadensewere domesticated in Central and South America
[20,22,23]. Wild cotton feature
short and coarse lint fibers that are very different from those
seen in current cultivar cottonseed. Although the elongated seed
trichomes may operate as a dispersion mechanism in some ecological
circumstances and/or function in maintaining an adequate microbiological
and hydration background for seed germination or early seedling
development, the biological function of lint fiber has not been
determined [24]. Early
domesticators were likely drawn to primitive lint fiber, with current
germplasm’s long lint fiber, resulting from the human selection of
genotypes with enhanced fiber quality attributes, as well as higher lint
output and other agronomic qualities
[23,25]. Interestingly, the
elongation of fiber cells during development is directly linked to the
evolution of long spinnable lint fiber; particularly, developing fiber
cells in domesticated diploid and tetraploid cotton all exhibit a
protracted fiber-cell growth programmed. Only the F-genome/A-genome
lineages developed this developmental novelty, which may have aided the
domestication of A-genome cotton
[26,27].
When the A-genome joined the D-genome during polyploidization, this
predisposition for extended lint fiber growth was passed on to
allopolyploids [28]. G.
hirsutum is said to have been domesticated in Mesoamerica’s Yucatan
Peninsula. According to Brubaker and
[29], race ‘punctatum’ is the
earliest domesticated form of G. hirsutum , having agronomic
characteristics that are transitional between the really wild race
‘yucatanense’ and races with more advanced attributes such as latifolium
and palmeri. Early attempts at domestication may have involved choosing
a more appealing plant from the wild population to generate door-yard
cultigens, which later evolved into large-scale field production as
civilization grew and agriculture became more specialized. Increased
lint output through choosing larger and more bolls per plant, lowering
plant size from the shrubby/small treelike habit of wild cotton to a
scale that people can utilize, and selecting for a more annual life
cycle are all qualities that early agriculturalists may have preferred.
Later decisions concentrated on finer, stronger, and consistently longer
fibers to improve the fiber’s quality. Other fiber qualities such as
elongation and short fiber content have grown increasingly important as
the textile industry has become more automated. As agriculture and
agribusiness converge, the industrialization of production will continue
to define aspects that increase production efficiency. For example,
growing mechanization needed plant size selection, but monoculture’s
vulnerability to pests and diseases necessitated enhanced selection for
resistance to these challenges. Ware
[30] has published a thorough
historical account of the history of G. hirsutum as Upland cotton
cultivars from the time of European arrival to the middle of the
twentieth century. The birth and development of Upland germplasm, the
current cultivated form of G. hirsutum , occurred in the southern
United States of America, despite the fact that the specific location ofG. hirsutum domestication is unclear. The ‘Cotton Belt’ of the
United States was the epicenter of upland cotton’s genetic development.
When a result, the cotton crop was brought to the eastern coastal areas
of North America as Europeans departed. While all four domesticated
Gossypium species were planted as crops in the United States from the
start, the allotetraploids outperformed the A genome diploids. As the
twentieth century progressed, a new categorization system based on
geographic areas and industrial techniques emerged, which is still in
use today: Acala type, Plains type, Delta type, and Eastern type
[31]. Asiatic diploid speciesG. arboreum and G. herbaceum , with much shorter staple
lengths than Upland or Sea Island cotton, were produced in
cotton-producing nations such as India, China, and Russia prior to the
establishment of the US cotton industry
[32]. The new spinning methods
available at the time could not handle the fiber produced by G.
arboreum and G. herbaceum . While there has been some success in
breeding for longer fiber length within the diploid species
[33], more Upland germplasm was
introduced to meet the need for new varieties with better fiber quality.
By the 1920s, practically all worldwide breeding efforts had shifted to
the allopolyploid G. hirsutum , a short-fiber Asiatic species
[30].
4. Cotton Improvement
Since Niles and Feaster’s report, the great majority of cotton breeding
efforts have continued to use the same technique [9]. Cotton is
mostly a self-pollinated plant; hence most cotton growers use a modified
pedigree breeding approach to generate pure line cultivars. In general,
parents with different attributes or traits of interest are chosen for
cross hybridization, segregating populations are evaluated in the field
to identify individual plants with the desired trait combinations, seed
from the selected plants is evaluated in progeny rows, and inbred lines
that outperform “check” cultivars are evaluated in replicated tests
over multiple locations and years
[34,35].
The procedures of cotton breeding programs have been nicely laid out in
literature
[36–38].
Current breeding operations make use of substantially more machinery,
allowing them to handle a bigger number of progeny rows with fewer
people and more sophisticated fertilization, plant growth control, and
pest management. Although the relative significance of these features
may have altered, the traits that are wanted in present cultivars are
not significantly different from those in earlier years of cotton
cultivation [37]. In every commercial cotton breeding program, lint
output remains the top objective
[39–41].
Lint percent, which is a component of lint output, was probably one of
the first qualities to be chosen throughout the domestication process
and in early breeding, and is still the most desired character
[23,42–46].
Environmental stability
[47,48]
and early maturity
[49–51]
are two more agronomic features that have risen in relevance [37].
Despite the fact that the advent of transgenic Bt cultivars has shifted
the attention away from insect resistance, host-plant resistance remains
significant for a variety of diseases and nematodes. Similarly, breeding
efforts used to priorities cotton genotypes with low vegetative growth
and erect stature, but plant height and compact growth habits may now be
readily regulated with plant growth regulators such as Mepiquat
[52]. The second most
significant aim in early breeding attempts was fiber quality, which is
still the case in commercial cotton breeding projects
[53–55].
The replacement of manual spinning methods with machine spinning and
weaving technologies, which required adequate fiber length and strength
to work properly, provided the motivation for this endeavor. Cotton
fibers’ quality is determined by their physical characteristics. Lint
fiber is frequently spun into yarn, which is subsequently woven or
knitted into a variety of textiles dependent on the quality and desired
end-product characteristics. The collection of fiber characteristics
that determine the efficiency of yarn spinning, weaving, and other
fabric-making activities, as well as the quality of cotton textiles, is
referred to as fiber quality. The key fiber factors that are
substantially linked with spinning performance and end product quality
are the length, strength, elongation, and fineness/maturity (measured in
micronaire) of the fiber. The relevance of fiber characteristics and how
they are quantified was nicely outlined by Chee et al. (2009)
[56].
Lint production and fiber quality are both quantitatively inherited
characteristics. For yield and fiber quality variables, Campbell et al.
[57] reported the mean
broad-sense and narrow-sense heritability. Although yield components and
fiber quality attributes are both heritable and exhibit additive genetic
variation and frequently exhibit a negative correlation, it is suggested
that this is due to linkage rather than pleiotropy; however, Campbell et
al. [58] found that the negative
connection in Pee Dee germplasm maintains after over 80 years of
breeding. As a result, improving yield and fiber quality simultaneously
is the most difficult problem in cotton breeding.
Meredith and Bridge (1973) tested the performance of four cottons
(G. hirsutum L. ) cultivars under four environmental
conditions and estimated seven yield components with seven fiber
properties using nine different harvests. Results revealed that the lint
index was lowest for early harvest and was highest for the middle
harvests, while cultivars were the most important source of variations
for fiber properties, indicating the importance of genetic variation for
fiber quality traits improvements
[59]. Later they used a modified
recurrent selection method for improving lint percentage within a cotton
(G. hirsutum L) cultivar ‘Deltapine 523,’ through initially
plant-based selection, followed by progeny-row basis selection and
construction of S1, S2, and S3 selfed generations. They finally attained
eight progenies in S3 with 2.5% higher span length
[60], thereby significantly
improving the cotton for lint yield, lint percentage, fiber length, lint
index, and Micronaire in the S3 generation population compared to the S0
population [60]. Using G. barbadense as a donor parent,
reciprocal backcross population of the S6 generation of G.
hirsutum × G. barbadense crosses showed significant genotypic
variations, and improvements in fiber-quality-related traits, including
micronaire, fiber elongation, fiber strength, and upper half mean length
showed significant higher genotypic variance in G. hirsutumbackground than G. barbadense, indicating cytoplasmic effects on
the genetic variations and heredity
[61]. The growing number of
evidences has been reported to verifying the adoptive roles of alien
introgression for different quality traits, especially fiber quality ofG. hirsutum[62–68].
5. Development of Spinnable Fiber and Polyploidization
In terms of fiber quality improvement, it is worth noting that spinnable
fiber appeared just once in Gossypium’s history, in an ancestor of the
two domesticated diploid A-genome species, after the F-genome lineage
split. This feature was handed down to allopolyploid cotton when the
A-genome fused with a D-genome from an ancestor that didn’t make lint
fibers in a shared nucleus
[27,69]. As previously stated,
the formation of long spinnable lint fiber is closely linked to a
protracted elongation phase during fiber cell development.
Applequist, Cronn and Wendel [26] used accessions from the AD-genome
allopolyploids G. hirsutum and G. tomentosum , as well as
diploid species G. herbaceum (A-genome), G. arboreum(A-genome), G. raimondii (D-genome), G. davidsonii(D-genome), G. anomalum (B-genome), and G. stur(F-genome). Accessions from the AD-genome allopolyploids and the
A-genome diploids have a much higher rate of fiber elongation than the
other diploids, according to a comparison of growth curves across
species. Hovav et al. [27] came to a similar result after comparing
gene expression profiles over a developmental time-course of fiber fromG. herbaceum and G. longicalyx . In domesticated A-genomeG. herbaceum , their findings revealed significant changes in the
expression of genes associated with stress responses and cell
elongation, as well as a longer developmental profile. Thus, the
evolution of lint fiber included a continuation of an ancient
developmental programmed that developed prior to polyploidization in the
ancestral A-genome. Polyploidization has another major meaning for the
evolution of spinnable fiber, in addition to supporting the evolution of
a protracted period of fiber elongation. The joining of the A- and
D-genomes in a single nucleus may have allowed D-genome alleles (genes)
to be recruited into fiber development, resulting in higher fiber
quality and production in polyploid cotton. Because only A-genome
diploid species generate spinnable fiber, the relevance of the D-genome
in polyploid cotton fiber quality genetic determination has long been
debated. Jiang et al. [70]
presented the first evidence of the extent to which loci on the
Dt-subgenome cause genetic variation in fiber quality attributes,
showing that the majority of QTLs for fiber quality mapping to the
Dt-subgenome. Numerous genetic mapping analyses, summarized by [56],
have now supported the observation that the Dt-subgenome, from the
ancestor that did not have spinnable fiber, plays a large role in the
genetic control of polyploid cotton fiber growth and development. These
findings show that Dt-subgenome genes have been recruited to the genetic
regulation of fiber quality attributes, leading to polyploid cotton’
transgressive fiber quality and yield compared to diploid progenitors.
Many advantageous alleles at important loci for fiber qualities may have
already been fixed as a result of natural selection, according to
[70], because the At-subgenome has a significantly longer history of
selection for fiber formation. On the other hand, fiber growth loci on
the Dt-subgenome may not have been subjected to strong selection until
after polyploidization, and hence, mutations that improved this feature
may have only been advantageous after polyploidization. As a result, the
Dt-subgenome may have had greater ‘room for improvement’ of fiber
qualities when the artificial selection was recently enforced by
domestication and breeding. Recruitment of Dt-subgenome loci may provide
polyploid cotton more flexibility for artificial selection through
breeding, explaining why polyploid cotton has better fiber
characteristics than farmed A-genome diploids. The uneven pace of
evolution in the polyploid AD-subgenome is a secondary consequence of
polyploidization for spinnable fiber and a host of other characteristics
that geneticists are only beginning to understand through genome
sequence analysis. The majority, if not all, loci are duplicated in
allopolyploid genomes by definition. Lynch and Conery
[71] addressed the different
outcomes of duplicated genes, claiming that most duplicated genes go
through a brief period of relaxed selection before being silenced or
pseudogenized. However, a tiny percentage of duplicated genes survive in
duplicate and contribute to developing phenotypic complexity through
natural selection. A null hypothesis is that homoeologous genes would
develop independently and at comparable rates following polyploid
formation in recent allopolyploids such as cotton, where the two
subgenomes contain duplicated but somewhat divergent copies of most
genes. Allopolyploid cotton have gone through bi-directional concerted
evolution for some genes, resulting in differing directional biases in
different sections of the genome. In situ hybridization revealed that
distributed repetitive sequences that are A-genome specific at the
diploid level had expanded to the Dt-subgenome in allopolyploids, which
provided the first clues to this occurrence
[72,73].
Furthermore, multiple studies have found that the Dt-subgenome has much
greater allelic diversity of homoeologous genes
[74] and loci affecting
quantitative features than the At-subgenome
[18,75]. Non-reciprocal DNA
conversion favors genes in the Dt-subgenome over genes in the
At-subgenome, according to recent genome sequence comparisons
[76]: for example, the sequences
of around 40% of the At and Dt genes in an elite cotton cultivar change
from their diploid ancestors. Most of these mutations are convergent,
with at least one gene being changed to the Dt state at a rate more than
double that of the reciprocal [9]. These findings show that
polyploidization allowed D-genome genes to take on new tasks in the
allopolyploid genome, potentially explaining why domesticated
allopolyploid cotton outperforms its diploid offspring in terms of
agronomic and fiber quality.
6. Gene Introgression and Inter-Specific Hybridization
Each phase in the domestication of cotton, from wild G. hirsutumto feral cultigens to the advent of Upland germplasm to current better
cultivars, imposed severe genetic bottlenecks, limiting allelic
diversity. Morphological [77],
and molecular characterization have been used to record the amounts and
patterns of genetic erosion involved with the formation of early
cultigens, landraces, and current cultivars
[78,79].
Too many genomic region and genes have reported to be introgressed from
wild relative of upland cotton contributed to the improvements of upland
cotton [8,65,80].
The average genetic distance across 378 Upland accessions from the
United States examined with 120 SSR markers was only 0.195,
demonstrating that Upland cotton germplasm is quite limited [79].
Lubbers and Chee [81] used 250
RFLPs to examine 320 Upland cultivars/germplasm from the United States
National Plant Germplasm Collection and found cotton to have less
genetic diversity than most important crops. When top germplasm from
various geographical origins was assessed, the amount of genetic
diversity did not improve. Indeed, the average number of alleles
discovered per locus in a survey of 157 elite cultivars from China, the
United States, Africa, the Former Soviet Union, and Australia utilizing
146 SSR loci was just 2.3 [82].
Surprisingly, these genetic limitations were followed by sustained
improvements in several key cotton properties, particularly fiber
quality. Given the low allelic diversity in the Upland cotton gene pool,
it is reasonable to assume that the number of favorable alleles for
fiber quality (such as fiber length and strength) that have yet to reach
fixation is small, as these traits have been under intense selection
pressure since the early stages of domestication. As a result, it is not
unexpected that interspecific introgression has long been a topic of
discussion in the Upland cotton community
[83,84].
Upland cotton breeding has prioritized high production and adaptability,
whereas domesticated strains of G. barbadense , often known as
Pima, Egyptian, or Sea Island cotton, have prioritized improved fiber
quality. As a result, farmed G. barbadense fiber is substantially
longer, finer, and stronger than Upland cotton, which is more
extensively grown. However, both Pima and Egyptian cotton have a
restricted range of environmental adaptability in irrigated regions in
dry zones of the Western United States and Lower Egypt, respectively, of
the G. barbadense that are still in production. Nonetheless, this
species’ distinctive fiber qualities make it an attractive option for
supplying additional genetic variety to increase Upland cotton fiber
quality. It is no surprise, therefore, that studies of populations
produced from interspecific hybridization between wild and domesticated
strains of G. barbadense and Upland cotton have investigated the
genetic basis and heritability of species fiber qualities
[56,85]. Interspecies genome
merge provides an opportunity to the introduce a foreign beneficial gene
for crop improvements and genetic analysis. Saha et al. (2006) developed
monosomic and monotelodisomic substitution hybrids between G.
hirsutum and G. tomentosum and identified several types of
numerical and structural variations and offered a valuable germplasm for
localization of genomic markers and development of backcross
substitution line for cotton cultivars’ improvements
[86]. Recently, Muthuraj et al.
(2019) developed male sterile triploid interspecific hybrids between
tetraploid G. hirsutum and diploid G. armourianum, which
showed intermediate phenotypes, and this germplasm is an important
genetic source for introducing sucking cotton pest “jassid”-resistant
genes into the cultivated cotton cultivars through conventional breeding
schemes [87]. In order to
barrier free wild gene introgression into cultivated cotton, a
tri-species hybrid “(G. arboreum × G. anomalum ) ×G. hirsutum ” was produced. The cytomorphological analysis of a
tri-species hybrid and its backcross progenies to G. hirsutumshowed the production of monovalent to hexavalent offspring and
allosyndetic chromosomes pairing, indicating the possibility of
intergenomic genetic exchanges and yet a homoeologous relationship among
these species [88]. As expected,
the molecular marker data combined with cytogenetic findings determined
the multi-genome background in monovalent to hexaploid progenies and
provided an important intermediate material for introducing exotic
genetic introgression [88]. Draye et al. (2005) used
backcross-self-pollination population of a G. hirsutum andG. barbadense cross and identified 32 and 9 QTLs for fiber
fineness and micronaire, respectively, and from nine micronaire QTLs,
seven were also associated with fiber fineness; however, the majority of
the members of the BC3F2 population showed inferior phenotypes, thus
imposing hurdles to utilize G. barbadense in conventional
planting breeding programs [89].
The breeding utilization of G. barbadense -introgressed line inG.hirsutum showed high mid-parent heterosis for yield, and F1 to
F3 hybrids outcompeted the high-yielding commercial cultivar
[90], indicating the suitability
of introgressed lines for being outcompeting cultivars. Remarkable
progress was made by Hulse-kemp et al. (2015) by developing
CottonSNP63K, an Illumina Infinium array with 45,104 intraspecific and
17,954 interspecific putative SNP markers, and generating two
high-density genetic maps, collectively providing new cotton resources
for cotton breeders [91]. Later,
Hinze et al. (2017) used this CottonSNP63K array and validated that it
could distinctly separate G. hirsutum from other Gossypiumspecies, distinguish the wild from cultivated types of G.
hirsutum , and identify loci possibly linked to cotton seed protein
contents [92]. As chromosome
segment substitution (CSSLs) lines provide an ideal opportunity to map
QTLs in interspecific hybrids, a CSSL derived by hybridizing and
backcrossing G. hirsutum and G. barbadense genotyped by
whole genome re-sequencing identified 64 QTLs for 14 agronomic traits,
and many alleles of G. barbadense showed extremely high values
for improving cotton seed pool contents
[93]. Recombinant inbred line
populations produced between the Chinese G. barbadense cultivar
5917 and the American Pima S-7 were tested for lint yield and fiber
quality traits, followed by GBS sequencing, and there were 42 QTLs
identified, including 24 QTLs on 12 linkage groups for fiber quality and
18 QTLs on 7 linkage groups for lint yield, thereby proving an initial
material for fine mapping of QTLs, prediction of candidate genes, and
development of molecular markers.
The majority of these genetic investigations have shown that fiber
qualities are heritable
[94–96],
implying that interspecific introgression might increase specific Upland
cotton fiber traits. In addition to fiber quality, wild and domesticated
allotetraploid Gossypium is a significant source of disease and pest
resistance genes that might be transferred into Upland cotton
[65,97–99].
Pathogens, nematodes, and insects cause severe crop losses anywhere
cotton is grown, and crop protection expenditures account for a large
percentage of the high unit cost of cotton production, which is why
transgenic pest-management cultivars are so appealing. According to
Meredith Jr [100], breeding for
disease resistance is more important than breeding for pest resistance
in most breeding programs. This is especially true with the development
of Bt cotton varieties, which are insect-resistant. Some wild species
resistance characteristics are simply inherited, and breeders have taken
advantage of these features since they are easy to select. However, many
resistance qualities are quantitatively inherited, and using DNA markers
to manipulate them has lately become considerably more successful.
Disease resistance genes found in wild and domesticated allopolyploid
Gossypium are summarized in Table 1. After polyploid development during
the Mid-Pleistocene, around 1–2 Mya, G. hirsutum , G.
barbadense , and the wild allopolyploid species separated from a common
ancestor. When introduced into a diverse genetic background, mutations
that have accumulated in various allopolyploid lineages may or may not
interact positively (Orr 1995). While allopolyploid species are sexually
compatible, later-generation hybrids sometimes exhibit partial
reproductive obstacles such as lower fertility, segregation distortion
(non-Mendelian inheritance), and hybrid breakdown
[101]. Jiang et al.
[102] employed DNA markers to
analyze the transmission genetics of an advanced back cross generation
interspecific hybrid population between G. hirsutum and G.
barbadense , demonstrating the effects of these barriers to gene
introgression across allotetraploid cotton. Individual allele
transmission patterns often promote the eradication of the donor
genotype, maintaining the recurrent genotype’s integrity. Segregation
distortions were common, for example, and early generations of
hybridization resulted in the full eradication of certain donor alleles
as early as the BC3 generation due to under-representation of donor
alleles. Interestingly, the segregating ratios at identical loci
introgressed into different independently derived BC3 F2 families were
highly variable, with some families favoring recurrent parent alleles
while others favor donor parent alleles, implying that hybrid
incompatibility is best explained by multi-locus epistatic interactions
affecting gamete success and genotype fecundity. These findings
illustrate the challenge of using interspecific populations to generate
superior Upland cotton cultivars by pyramiding numerous beneficial
alleles for quantitative features, including lint production, fiber
length, fiber strength, and fiber fineness in a single genotype. Most
attempts to directly mix Upland cotton with Sea Island (G.
barbadense ) varieties, as observed by Brown and Ware
[103], revealed that while the
F1 generation is beautiful, the F2 and F3 generations are regarded as
“messy” and are nearly usually abandoned (Figure 2). While pedigree
research revealed that several cultivars were created through
interspecific hybridization, a molecular study using isozymes and DNA
markers revealed that the Upland cotton gene pool is rather uniform.
Rare alleles are found in just a few closely related cultivars within a
germplasm group and are thought to have evolved by introgression
[6,104]. Furthermore, none of
these G. barbadense -introgressed alleles were discovered in
current cotton cultivars, suggesting that the advantages of G.
barbadense introgression in Upland cotton are still completely unmet.
Conversely, the introduction of Upland cotton genes into Pima cotton, a
cultivated variety of G. barbadense , has substantially aided the
production and adaptability of current Pima cultivars
[105,106].
Figure 2. Floral abnormality in later generation hybrids
between Gossypium hirsutum and G. barbadense : (A )
abnormal style, stigma, and anther formation; (B ,C )
abnormal bud development; and (D ) an abnormal flower of an F2
generation of Gossypium hirsutum × G. barbadense .
7. Introgressive Breeding
Interspecific populations derived from crossings between G.
hirsutum and G. barbadense solved the classic problem of limited
genetic diversity in Upland cotton genetic and QTL mapping in the early
days. These two grown species are sought for somewhat different traits
in addition to providing the DNA-level polymorphism required for genetic
map creation. As previously stated, G. hirsutum breeding has
stressed maximal output and broad adaptability, whereas G.
barbadense breeding has emphasized fiber quality. As a result, most
genetic mapping [15,107] and
molecular quantitative genetic studies of fiber properties have used
populations derived from interspecific hybridization involving wild and
domesticated forms of G. barbadense crossed with Upland cotton.
QTLs for numerous fiber quality variables mapped in cotton were
summarized by Chee and Campbell [57]. Some important QTLs for fiber
length, strength, and fineness have now been found, and several have
been verified
[108,109].
This knowledge gives cotton growers greater options to improve certain
fiber qualities in upland cotton by introducing genes from G.
barbadense with minimal disruption to the favorable allelic
combinations developed over a century of selective selection. Inbred
backcross populations generated from crossing Upland varieties with the
allotetraploid species G. barbadense , G. tomentosum , andG. mustelinum have been developed as part of a collaborative
effort to minimize Upland cotton’s genetic susceptibility
[110]. Inbred backcrossing was
used to reduce reproductive obstacles caused by interspecific
introgression between these species
[111,112].
One can examine very tiny pieces of introgressed DNA for agronomic or
fiber quality performance and analyze them for QTLs by establishing a
comprehensive set of Near Isogenic Introgression lines from the BC2 or
BC3 families. Because recombination and segregation have split the donor
genome into smaller components, the activity of specific genetic loci
may be more clearly defined than in previous generations. Table 2 shows
the number of QTLs detected in each of the inbred backcross groups.
Rong, Feltus, Waghmare, Pierce, Chee, Draye, Saranga, Wright, Wilkins
and May [18] reported the alignment of 432 fiber QTLs identified in
10 interspecific G. hirsutum by G. barbadense populations
into a consensus map, providing further information on the genetic
dissection of each of the fiber properties.
Table 2. Summary of QTL mapped in interspecific Gossypium
species.