Research Article |
Corresponding author: N’guessan Olivier Konan ( nguessanolivier@yahoo.fr ) Academic editor: Elena Mikhailova
© 2020 N’guessan Olivier Konan, Guy Mergeai.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Konan NO, Mergeai G (2020) Relationship between meiotic behaviour and fertility in backcross-1 derivatives of the [(Gossypium hirsutum × G. thurberi)2 × G. longicalyx] trispecies hybrid. Comparative Cytogenetics 14(1): 75-95. https://doi.org/10.3897/CompCytogen.v14i1.47231
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Wild cotton species are an important source of desirable genes for genetic improvement of cultivated cotton Gossypium hirsutum Linnaeus, 1763. For the success of such an improvement, chromosome pairings and recombinations in hybrids are fundamental. The wild African species G. longicalyx Hutchinson & Lee, 1958 could be used as donor of the desirable trait of fiber fineness. Twelve BC1 plants obtained from the backcrossing of [(G. hirsutum × G. thurberi Todaro, 1877)2 × G. longicalyx] (AhDhD1F1, 2n = 4x = 52) trispecies hybrid (HTL) by G. hirsutum (cv. C2) (AhAhDhDh, 2n = 4x = 52) were investigated for meiotic behaviour and plant fertility. Their chromosome associations varied as follows: (2.5 to 11.5) I + (17 to 22) II + (0.31 to 1.93) III + (0.09 to 1.93) IV + (0 to 0.07) V + (0 to 0.14) VI. Their pollen fertility ranged from 4.67 to 32.10 %. Only four BC1 plants produced a few seeds through self-pollination. The remaining BC1 were totally self-sterile and usually presented the highest number of univalents. All BC1 materials produced BC2 seeds (0.44 to 6.50 seeds per backcross) with the number of seeds negatively correlated with the number of univalents (R2 = 0.45, P < 0.05). Most BC1 plants gave significantly finer fiber compared to the cultivated G. hirsutum. SSR markers showed a segregation of wild alleles among the backcross derivatives and Genomic in situ hybridization (GISH) revealed presence of entire chromosomes of G. longicalyx as well as recombinant chromosomes in the backcross derivatives. The significance and details of these results are presented and the prospects of successfully exploiting these plant materials are discussed.
chromosome, cytogenetics, fiber fineness, Gossypium spp, hybrid, in situ hybridization, meiosis, plant breeding
Cotton is the most important fiber crop in the world. It belongs to the genus Gossypium which comprises about 53 species (
Only four cotton species are cultivated, of which G. herbaceum Linnaeus, 1753 (A1 genome) and G. arboreum Linnaeus, 1753 (A2 genome) are diploid, while G. hirsutum Linnaeus, 1763 ((AD)1 genome) and G. barbadense Linnaeus, 1753 ((AD)2 genome) are tetraploid (
In cotton breeding, wild species are an important source of several desirable genes for genetic improvement of G. hirsutum such as fiber quality, resistance to diseases and insect pests, or tolerance to abiotic stress. The wild species G. longicalyx Hutchinson & Lee, 1958 (F1 genome) could be used as donor of the desirable traits of fiber fineness, length and strength, which are very important to textile industry (
In interspecific breeding programs, carrying out continuous cytological analysis is very important for plant selection because it provides information concerning the degree of meiotic irregularities, viability of gametes, chromosome pairing and genetic recombination (
Another responsible factor for the success of breeding is the selection of genotypes with a high percentage of viable gametes (
The observations of both meiotic behaviour and plant fertility can thus help reducing the time needed for producing new hybrid cultivars, since plants with meiotic irregularities and/or unviable pollen grains can be rejected for selection of more stable genotypes (
The objective of this study was to develop backcross progenies from the HTL hybrid, and to analyse their meiotic behaviour and their fertility with the long-term objective of introgressing the improved fiber fineness trait from G. longicalyx into G. hirsutum.
[(Gossypium hirsutum cv. C2 × G. thurberi G27)2 × G. longicalyx G17] (AhDhD1F1, 2n = 4x = 52) trispecies hybrid created by
Twenty seven BC1 seeds were hulled and cultivated in vitro on Murashige and Skoog medium (
Cytological analyses on the plant material produced were performed on their pollen mother cells (PMC) at meiosis. Suitable flower buds of each plant were collected between 09:00 and 11:00, according to the weather conditions, and fixed in fresh Carnoy’s II fluid (glacial acetic acid 1: chloroform 3: and ethanol 6) for 72 hours at 4 °C. They were then stored at 4 °C in 70 % ethanol until their evaluation. To obtain meiotic plates, a few anthers were squashed in a drop of 1.5 % acetic-carmine solution on a microscope slide, debris were removed with fine forceps, and the slide was covered with a coverslip and heated a few seconds over a flame to improve chromosome staining. With pressure on the coverslip, pollen mother cells were flattened to spread out chromosomes. Observations were made with a Nikon Eclipse E800 photomicroscope (Nikon, Tokyo, Japan) under oil immersion. We concentrated our observations on metaphase I stage where chromosome arrangements (univalents, bivalents, and multivalents) were counted. But meiotic abnormalities such as laggard chromosomes at Anaphase I, Telophase I, Metaphase II, Anaphase II and final abnormal products of meiosis were considered as well.
To have an indication of pollen quality, about 300 pollen grains per plant were analyzed. Flowers were collected in the morning on the day of anthesis. Pollen grains were dipped in a drop of 1.5 % acetic-carmine solution on a slide for 30 minutes and were analysed under a stereomicroscope Nikon Eclipse E800 (Nikon, Tokyo, Japan). Only fully stained and large pollen grains were scored as viable and non-aborted. The quantity of viable pollen was estimated as the percentage of stained pollen.
The self-fertility of the BC1 plants was assessed by determining the average number of seeds obtained from 30 self-pollinated flowers of each BC1 genotype. The cross-fertility of the BC1 plants was assessed by counting the average number of BC2 seeds obtained per backcross.
For fiber fineness analysis, the fibers were combed and a tuft of parallel fibers was cut from the seed. Their free points were also cut and the median region was placed on a slide and covered with a cover glass. We let one or two drops of 18 % NaOH solution penetrate by capillarity into the fibers. The NaOH solution swells the fibers. The diameter of at least 100 fibers was then measured with the software NIS-Elements BR 2.30 using the Nikon Eclipse E800 microscope (Nikon, Tokyo, Japan) equipped with a digital JVC KY-F 58E camera (JVC, Yokohama, Japan). The ribbon width was determined by dividing the mean of the diameters measured by the 1.3 Summers coefficient (
SSR marker analysis was achieved to check molecular segregation among the BC1 plants. Total genomic DNA was isolated from young fresh leaf tissues following the mixed alkyltrimethylammonium bromide (MATAB) method described by
Polymerase chain reactions (PCR) were performed in 10 μL volume containing approximately 25 ng of template DNA, 0.6 U of Taq DNA Polymerase, 2.5 mM MgCl2, 1× Polymerase Buffer, 2 µM of each forward and reverse primers, and 0.2 mM of dNTPs mix. A PTC-200 thermal cycler (BioRad, Belgium) was used, with a PCR conditions consisting of an initial denaturation at 94 °C for 5 min, followed by 35 cycles of denaturation at 94 °C for 30 s, annealing at 55 °C for 1 min and extension at 72 °C for 1 min, with a final 72 °C extension for 8 min. Amplification products were separated on 6.0 % denaturing polyacrylamide gel and visualized by silver stain according to the protocol of
To know whether the meiotic behaviour observed in the HTL trispecies hybrid and its BC1 derivatives allows recombination between G. hirsutum and G. longicalyx chromosomes, genomic in situ hybridizations (GISH) were performed using fast-growing root tips of twelve BC2 seeds belonging to the progeny of a randomly chosen BC1 plant.
DNA probe labelling
Total genomic DNAs were labelled by nick translation method with digoxigenin-11-dUTP (Roche, Switzerland) and biotin-14-dATP (Invitrogen life technologie, Carlsbad, USA) according respectively to the labelling protocol of the manufacturers. To reveal chromosomes or chromatin of G. longicalyx in the HLT hybrid and in BC2 derivative, Digoxigenin-11-dUTP was used to label total genomic DNA of G. longicalyx and biotin-14-dATP to label total genomic DNA of G. hirsutum.
Chromosome preparation
For mitotic metaphase chromosomes preparations, fast-growing root tips were collected in 0.04 % 8-hydroxyquinoline for 4 hours at room temperature (RT) and fixed for 48 h in a fresh fixative fluid (3:1 ethanol: acetic acid) at 4 °C. After washing in distilled water (2 × 10 min), treating in 0.25 N HCl (10 min), rinsing in distilled water (10 min) and treating in a 0,01M citrate buffer (10 min), root tips were digested in an enzyme solution (5 % cellulase Onozuka R-10, 1 % pectolyase Y-23 in citrate buffer) at 37 °C for 1 hour. The enzyme mix was removed by rinsing in distilled water for 10 min, and on a clean glass slide a single root tip was spread in one or two drops of fresh fixative (3:1 ethanol : acetic acid) using a fine-pointed forceps. Slides were stored at -20 °C until needed.
In situ hybridization
In situ hybridization was performed according to the protocol used by
Hybridization signal detection
After hybridization, to dissociate non-specific and imperfect hybrids, posthybridization stringent washes were performed successively in 2 × SSC, 0.5 × SSC, 0.1 × SSC, for 10 min each wash at 42 °C and in 2 × SSC for 10 min at 37 °C. Slides were afterwards incubated with 200 µl (per slide) of 5 % BSA-4SSC/Tween for 10 min at 37 °C. Both the BSA (bovine serum albumin) and the detergent Tween bind to the unoccupied sites, preventing subsequent non-specific binding of the antibodies. Biotin detection with Texas Red, digoxigenin with FITC (fluorescein isothiocyanate) and amplification were achieved as follows: Slides were incubated for 45 min at 37 °C three times; the first time with 50 µl of 5µg/ml Texas Red-avidin in TNB (100 mM Tris HCl (pH 7.5), 150 mM NaCl, 0.5 % Blocking reagent), the second time with 50 µl of a mixture of 12.5 µg/ml Biotinilated anti-avidin + 2 µg/ml anti-Dig FITC in TNB and lastly with 50 µl of a mixture of 5 µg/ml Texas Red-avidine + 5 µg/ml FITC-conjugated rabbit anti sheep in TNB. Each incubation was followed by two washes in TNT (100 mM Tris HCl (pH 7.5), 150 mM NaCl, 0.05 % Tween 20) at 37 °C for 5 min. The slides were wash for 1 min in 2 × SSC at 37 °C and dehydrated in an ethanol series of 70 %, 95 %, 100 % for 1 min at RT. Chromosome preparations were counterstained with DAPI (4,6-diamidino-2-phenylindole) in Vectashield. Slides were examined with an epifluorescence Nikon Eclipse E800 microscope (Nikon, Tokyo, Japan) using appropriate filters and a JVC KY-F 58E camera (JVC, Yokohama, Japan). Images were captured and processed with the softwares ArcSoft PhotoStudio 2000 4.3 and Adobe Photoshop 7.
On 183 backcrosses achieved with the HTL hybrid and G. hirsutum, only 27 BC1 seeds (i.e. 0.15 seeds per cross) were obtained with generally one seed per boll. Thirteen of these seeds gave rise to viable plants. Among these plants, only twelve produced flower buds and could be submitted to cytogenetic analysis. An important segregation regarding morphological characters was observed among the BC1 plants. The plant heights ranged from 133 cm (BC1/10) to 241 cm (BC1/2), while the heights of G. hirsutum and the HTL were on average 150 and 274 cm respectively. The size of the BC1 plants leaves was variable but all of them were bigger than the leaves of G. thurberi and G. longicalyx, and close to those of G. hirsutum. The colour of the flowers was pale cream, light yellow or yellow. The BC1/10 plant had flowers with red spot at the base of the petals like G. thurberi. It was the sole BC1 plant that presented this trait. No pollen grains were noted on the anthers of the BC1/12 genotype. The anthers of this plant remained indehiscent even after the flower opened.
Table
Ribbon width of parental species and the BC1 progenies of the HTL trispecies hybrid.
Genotype | Number of fibers analysed | Ribbon widh (µm) ± standard deviation | LSD grouping |
---|---|---|---|
G. hirsutum (cv. C2) | 107 | 17.765 ± 0.130 | H |
G. thurberi | 107 | 15.769 ± 0.130 | I |
G. longicalyx | 113 | 5.940 ± 0.126 | A |
HTL hybrid | 120 | 12.649 ± 0.123 | B |
HTL BC1/1 | 112 | 16.235 ± 0.127 | J |
HTL BC1/2 | 111 | 16.276 ± 0.128 | J |
HTL BC1/3 | 122 | 13.414 ± 0.122 | D |
HTL BC1/4 | 110 | 13.039 ± 0.128 | C |
HTL BC1/5 | 108 | 15.347 ± 0.129 | H |
HTL BC1/6 | 124 | 13.336 ± 0.121 | CD |
HTL BC1/7 | 110 | 14.822 ± 0.128 | G |
HTL BC1/8 | 115 | 14.457 ± 0.125 | F |
HTL BC1/9 | 111 | 14.358 ± 0.128 | EF |
HTL BC1/10 | 111 | 14.081 ± 0.128 | E |
HTL BC1/11 | 114 | 13.41 ± 0.126 | D |
HTL BC1/12 | 111 | 14.327 ± 0.128 | EF |
All the SSR markers used revealed polymorphism between the parental species. They also showed segregation of the diploid species alleles among the BC1 plants (Table
Distribution of homozygous (hh) and heterozygous (hl, ht or hlt) for SSR marker BNL832, BNL3279 and BNL2662 in the BC1 progeny (h: allele of G. hirsutum; l: allele of G. longicalyx; t: allele of G. thurberi).
Genotype | BNL 836 | BNL3279 | BNL2662 |
---|---|---|---|
HTL BC1/1 | hh | hh | hh |
HTL BC1/2 | hl | hl | hl |
HTL BC1/3 | hl | hl | hl |
HTL BC1/4 | hh | hh | hh |
HTL BC1/5 | hl | hlt | hlt |
HTL BC1/6 | hl | hl | hl |
HTL BC1/7 | hh | hl | hl |
HTL BC1/8 | hh | ht | ht |
HTL BC1/9 | hh | hh | hh |
HTL BC1/10 | hh | hh | hh |
HTL BC1/11 | hh | hh | hh |
HTL BC1/12 | hh | hh | hh |
Polyacrylamide gel with BNL3279 SSR marker presenting allele segregations in the parental species, the hexaploid, the HTL tri-species hybrids and the BC1 plants. 1 G. hirsutum 2 G. thurberi 3 G. longicalyx 4 the hexaploid (G. hirsutum × G. thurberi)2 5–13 HTL tri-species hybrid plants (G. hirsutum x G. thurberi)2 × G. longicalyx 14–24 BC1 plants (HTL x G. hirsutum). Black arrow: allele of G. hirsutum, Blue arrow: allele of G. thurberi, Red arrow: allele of G. longicalyx.
HTL BC1 plants presented great variations among them regarding pollen grain shape, size and stainability, unlike the pollen of the HTL parental species (G. hirsutum, G. thurberi and G. longicalyx) which had uniform size and were easily stainable with acetic-carmine (Fig.
Cotton pollen grain viability revealed by acetic-carmine staining. A G. hirsutum viable pollen grains with good colour and uniform size B mixture of viable and non viable pollen grains of a [(G. hirsutum × G. thurberi)2 × G. longicalyx] backcross-1 plant. Scale bars: 100 μm.
Self-fertility of the first backcrossing (BC1) progeny of [(G. hirsutum × G. thurberi)2 × G. longicalyx] trispecies hybrid (HTL) and its parental species.
Genotype | Total number of examined pollen grain (% of stainable pollen grain) | No of self-fertilized flowers | No of aborted capsules | No of capsules harvested | No of seeds harvested | Average number of seeds per self-fertilized flower | |
---|---|---|---|---|---|---|---|
No of capsules without seeds | No of capsules containing seeds | ||||||
G. hirsutum (cv. C2) | 200 (100.00) | 30 | 0 | 0 | 30 | 1020 | 34 |
G. thurberi | 131 (100.00) | 30 | 0 | 0 | 30 | 450 | 15 |
G. longicalyx | 200 (100.00) | 30 | 0 | 0 | 30 | 180 | 6 |
HTL BC1/1 | 362 (15.19) | 30 | 26 | 4 | 0 | 0 | 0 |
HTL BC1/2 | 321 (4.67) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/3 | 348 (14.37) | 30 | 28 | 1 | 1 | 1 | 0.03 |
HTL BC1/4 | 383 (22.45) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/5 | 416 (11.78) | 30 | 26 | 1 | 3 | 6 | 0.2 |
HTL BC1/6 | 352 (23.39) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/7 | 405 (32.10) | 30 | 27 | 0 | 3 | 12 | 0.4 |
HTL BC1/8 | 383 (13.84) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/9 | 267 (9.36) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/10 | 332 (9.94) | 30 | 28 | 0 | 2 | 9 | 0.3 |
HTL BC1/11 | 335 (10.45) | 30 | 30 | 0 | 0 | 0 | 0 |
HTL BC1/12 | Indehiscent anthers | 30 | 30 | 0 | 0 | 0 | 0 |
Table
Backrcrosses | No of pollinated flowers | No of capsules harvested | No of seeds harvested | Mean number of seeds per backcross | |
---|---|---|---|---|---|
No of capsules without seeds | No of capsules containing seeds | ||||
HTL BC1/1 × G. hirsutum | 20 | 10 | 10 | 11 | 0.55 |
HTL BC1/2 × G. hirsutum | 33 | 0 | 33 | 92 | 2.79 |
HTL BC1/3 × G. hirsutum | 17 | 1 | 16 | 36 | 2.12 |
HTL BC1/4 × G. hirsutum | 43 | 20 | 23 | 42 | 0.98 |
HTL BC1/5 × G. hirsutum | 37 | 0 | 37 | 191 | 5.16 |
HTL BC1/6 × G. hirsutum | 23 | 13 | 10 | 22 | 0.96 |
HTL BC1/7 × G. hirsutum | 43 | 0 | 43 | 219 | 5.09 |
HTL BC1/8 × G. hirsutum | 74 | 8 | 66 | 157 | 2.12 |
HTL BC1/9 × G. hirsutum | 75 | 53 | 22 | 33 | 0.44 |
HTL BC1/10 × G. hirsutum | 56 | 7 | 49 | 364 | 6.5 |
HTL BC1/11 × G. hirsutum | 66 | 39 | 27 | 38 | 0.57 |
HTL BC1/12 × G. hirsutum | 21 | 12 | 9 | 10 | 0.48 |
Total | 508 | 163 | 345 | 1215 | 2.39 |
Meiosis studies were performed on the HTL BC1 progeny, and also on G. hirsutum as control. With the exception of the HTL BC1/12 plant which had 54 chromosomes, all the analyzed HTL BC1 plants had 52 chromosomes. At metaphase I, chromosomes of G. hirsutum paired perfectly with 26 bivalents (Fig.
Meiotic metaphase I plates in G. hirsutum and in the HTL BC1/4 plant. A Meiotic metaphase I cell showing 26 bivalents in control G. hirsutum B meiotic metaphase I cell showing 8 univalents (long arrows), 20 bivalents and 1 quadrivalent (short arrow) in HTL BC1/4. Scale bars: 5 μm.
Genotype | No of analyzed c ells | Chromosome configuration | Chromosome Number | Average No of chromosomes paired | |||||
---|---|---|---|---|---|---|---|---|---|
I | II | III | IV | V | VI | ||||
G. hirsutum | 10 | 26 | 52 | 52 | |||||
HTL BC1/1 | 30 | 6.90 (1–12) | 21.07 (17–24) | 0.53 (0–3) | 0.33 (0–2) | 0.03 (0–1) | 52 | 45.23 | |
HTL BC1/2 | 32 | 9.37 (4–16) | 20.66 (18–24) | 0.31 (0–2) | 0.09 (0–1) | 52 | 46.62 | ||
HTL BC1/3 | 30 | 5.23 (0–12) | 21.93 (16–26) | 0.43 (0–2) | 0.43 (0–2) | 52 | 46.90 | ||
HTL BC1/4 | 30 | 7.73 (4–12) | 20.67 (16–24) | 0.37 (0–2) | 0.40 (0–2) | 0.03 (0–1) | 52 | 42.20 | |
HTL BC1/5 | 12 | 4.75 (2–10) | 21 (19–24) | 0.75 (0–3) | 0.75 (0–2) | 52 | 47.25 | ||
HTL BC1/6 | 33 | 5.85 (2–10) | 21.67 (17–24) | 0.45 (0–4) | 0.36 (0–2) | 52 | 46.15 | ||
HTL BC1/7 | 30 | 3.83 (0–8) | 22.8 (19–25) | 0.37 (0–3) | 0.33 (0–2) | 52 | 48.03 | ||
HTL BC1/8 | 37 | 8.63 (4–14) | 18.94 (13–23) | 0.51 (0–4) | 0.97 (0–4) | 52 | 43.32 | ||
HTL BC1/9 | 21 | 10.38 (5–15) | 18.90 (15–22) | 1.14 (0–3) | 0.09 (0–2) | 52 | 41.62 | ||
HTL BC1/10 | 14 | 2.5 (0–5) | 22 (0–2) | 1.07 (0–2) | 0.57 (0–1) | 52 | 49.50 | ||
HTL BC1/11 | 32 | 11.50 (7–17) | 17.81 (10–22) | 1 (0–4) | 0.47 (0–3) | 52 | 40.50 | ||
HTL BC1/12 | 14 | 5.28 (0–8) | 17 (14–20) | 1.93 (0–6) | 1.93 (0–4) | 0.07 (0–1) | 0.14 (0–1) | 54 | 48.71 |
Unlike observations made in G. hirsutum, PMC meiosis in the BC1 progeny was mostly abnormal (Fig.
Meiotic aspect with abnormalities in HTL BC1 plants. A Leptotene B diakinesis C metaphase I with univalent chromosomes in early ascension (arrow) D anaphase I E telophase I with the presence of laggard chromosomes (arrows) F metaphase II with laggard chromosomes (arrows) G anaphase II with presence of laggard chromosome (arrow) H microsporocytes with a mixture of tetrad and polyads with micronuclei. Scale bars: 20 μm.
Figure
Genomic in situ hybridization on mitotic metaphase chromosomes of the HTL trispecies hybrid [(G. hirsutum × G. thurberi)2 × G. longicalyx)] and a BC2 progeny. A Mitotic metaphase showing 52 chromosomes of the HTL hybrid revealed by counterstaining with DAPI B mitotic metaphase showing in the HTL hybrid 13 green chromosomes from G. longicalyx, 13 yellow-orange chromosomes from the A-subgenome of G. hirsutum and 26 red chromosomes from G. thurberi and the D-subgenome of G. hirsutum revealed after the superimposition of FITC detection and Texas Red detection C mitotic metaphase showing 52 chromosomes in a HTL BC2 revealed by counterstaining with DAPI D mitotic metaphase in a HTL BC2 showing an entire chromosome of G. longicalyx (white arrow) and an intergenomic recombination (red arrow) showing movement of G. longicalyx chromatin into a chromosome of the A-subgenome of G. hirsutum. Scale bars: 5 μm.
Compared with the homologous chromosome pairing in the parental G. hirsutum species which showed only bivalents, the meiotic behaviour of HTL BC1 progeny was abnormal. In these plants, a disturbed meiosis was observed and, at metaphase 1, chromosomes paired imperfectly and gave, in addition to bivalents, some univalents and multivalents. The same abnormalities were observed by
Four BC1 genotypes were able to produce a few seeds through self-pollination. These plants which produced also the highest number of BC2 seeds per backcross were characterized by a better meiotic stability. Their relative lower univalent rate and higher paired chromosome rate could explain their higher self- and cross-fertility. Globally, the number of BC2 seeds produced per backcross was negatively correlated with the average number of univalents at Metaphase I (R = -0.67, P < 0.05) indicating a deleterious role of unpaired chromosomes on the fertility of the hybrid plants. This is in accordance with
However, irregular meiotic behaviour cannot be the sole explanation of the observed sterility problems. Indeed, some of the HTL BC1 plants presenting a few univalents were totally self-sterile, indicating that additional factors may reduce the actual gamete viability of the BC1 plants. The same observation was earlier made by
Although only four BC1 plants produced a few seeds through self-pollination, all the BC1 plants gave seeds by backcrossing when they were used as female. This result indicates that instead of the male sterility presented by most of BC1 plants, the potential for female reproduction remains.
Evaluation of fiber fineness showed that G. longicalyx had the finest fiber among the parental species, and the HTL trispecies hybrid and some of its BC1 progenies gave finer fiber than the main cultivated cotton G. hirsutum. This result confirms that G. longicalyx is a good donor for fiber fineness. The plants BC1/3, BC1/4, BC1/6, and BC1/11 which gave the lowest ribbon width are interesting genetic stocks which can be selected for further improvement of this trait in a breeding program. However, as one of the main success factors of breeding is the selection of genotypes with a high percentage of viable gametes (
SSR marker analysis revealed segregation of diploid alleles among the BC1 plants indicating the differential presence or absence of the diploid species chromosomes and/or chromosome recombinations. This allele segregation supports the segregation observed in BC1 plants regarding fiber fineness trait and the other morphological traits. For the success of an interspecific breeding program, homoeologous recombinations are crucial. Interspecific hybridization finds its justification in the possibility for genetic material exchanges between the genome of the different target species. The degree of homology between the parental genomes is fundamental for the occurrence of intergenomic recombination in successive backcross progenies. In general, chromosomes of closely related genomes tend to pair more often than chromosomes of genomes that are more distantly related (
Cotton fibers sustain one of the world’s largest industries, the textile industry, for wearing apparel, home furnishings, and medical supplies. Further improvement of cotton fiber quality is much desired. Most of the BC1 plants studied have presented an increased number of paired chromosomes compared to the parental HTL trispecies hybrid. Moreover, all of them could produce seeds through backcrossing and some were self-fertile. The plant material is gaining stability. Some BC1 plants exhibited interesting fiber fineness and recombination is possible between the donor species chromosomes and G. hirsutum chromosomes. These results are promising for the introgression of the improved fiber fineness trait from G. longicalyx into upland cotton.