The Journal of Heredity 2002:93(2)
© 2002 The American Genetic Association 93:130-132
Tests of Six Cotton (Gossypium hirsutum L.) Mutants for Association with Aneuploids
From the U.S. Department of Agriculture, Agricultural Research Service, Southern Plains Agricultural Research Center, Crop Germplasm Research Unit, 2765 F&B Road, College Station, TX 77845 (Kohel and Yu), and Texas A&M University, Soil Crop Science Department, College Station, TX 77843 (Stelly).
Address correspondence to R. J. Kohel at address above or e-mail: kohel{at}kapas.tamu.edu.
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Abstract |
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Genetic mutants are useful tools for basic and applied research to elucidate the developmental and regulatory processes of the cotton plant (Gossypium hirsutum L.). Their value is enhanced with knowledge of their location in the genome. The results of aneuploid tests used to locate mutant loci on specific chromosomes in G. hirsutum L. are reported. Thirty-four monosomes and telosomes, representing 18 of the 26 chromosomes, were used in combination with six mutants that were associated with nine loci. The mutant loci were glandless stem and boll (gl1gl6), immature fiber (im), Ligon lintless-2 (Li2), methylation (me), nonpinking (np1np2), and Raimondal (Ra1Ra2). We found that im was associated with chromosome 3 that contains linkage group VI (accessory involucre and frego bract); Li2 was associated with chromosome 18 that contains linkage group XVI (open bud and yellow pollen-2); and me was associated with chromosome 9. The remaining three mutants were not associated with the aneuploids in the tests. Knowledge of these chromosome assignments provides a valuable reference for specific studies of mutants and for further genome mapping efforts.
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Introduction |
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TopAbstractIntroductionDescription of Traits and...MethodsResults and DiscussionReferences |
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Crop genetics and breeding benefit from research on genetic mutants that qualitatively or quantitatively influence plant traits. The discovery or induction of mutants, as well as their characterizations, localization, and maintenance are important. Genetic and linkage analyses of mutants are part of the description process, and tend to be especially interesting in polyploid species such as cotton (e.g., Gossypium hirsutum L. and Gossypium barbadense L.), due in part to genetic redundancy and intergenic complementation between homologous loci.
In G. hirsutum, hypoaneuploid (haplo) stocks provide an effective means to associate the gene loci with specific chromosomes (Endrizzi et al. 1985). The core collection of hypoaneuploid cytogenetic stocks includes monosomic and monotelodisomic types collectively affecting 19 of the 26 chromosomes (Stelly 1993). For most chromosomes, the genetic redundancy in the polyploid genome of G. hirsutum allows for sporophytic hemizygosity and transmission of hypoaneuploidy via the megagametophyte. Multiple-marker genetic linkage test stocks afford the opportunity to associate loci with existing genetic linkage groups. Although the cytogenetic testers provide incomplete genome coverage of the cotton genome, the tests are simple and usually unequivocal. We report herein the results of tests involving six morphological, pigmentation, and biochemical traits (nine loci).
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Description of Traits and Loci |
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Glandless stem and boll (gl1) was first reported by McMichael (1954) as a single recessive gene mutation. Murray (1965) later recovered a variant determined to be gl1Gl6, and it was recognized that most cottons are recessive at the gl6 locus. So most tests of glandless stem and boll test only the gl1 locus. Tests of gl6 involve the use of a gl1Gl6 line. Tests in synthetic hybrids by Gerstel and Phillips (1958) suggested that gl1 is in the A subgenome.
Two of the loci affect fiber development. The immature fiber (im) mutation affects the seed-borne fibers as a simple recessive mutant (Kohel and McMichael 1990). Mutant plants are recognized by their nonfluffy cotton fiber resulting from extremely fine, low-micronaire fibers. Ligon lintless-2 (Li2) is a completely dominant single-gene mutation that has extremely short fibers, less than 10 mm (Narbuth and Kohel 1990).
The gene for methylation (me) increases methylation of terpenoid products in cotton. It was introduced into G. hirsutum from G. barbadense (Bell et al. 1994) and shown to be inherited as a simple recessive in G. hirsutum (Kohel and Bell 1999). Raimondal, which is a unique terpenoid form, was transferred from the 26-chromosome Peruvian species G. ramondii Ulb. (Bell et al. 1994; Stipanovic et al. 1994). Genetic analysis (Kohel and Bell 1999) determined that dominant genes at two loci (Ra1Ra2) acted epistatically to produce the Raimondal phenotype. Since the trait was transferred from a D-genome cotton, one or both of the loci are in the D subgenome of G. hirsutum.
Nonpinking was reported by Rhyne and Carter (1991) as a duplicate recessive mutant (np1np2). Mutant nonpinking plants lack anthocyanins, and they appear lighter green in color than normal cotton.
No linkages were found between these mutant loci and the markers with which they were tested (Percy and Kohel 1999). The mutant lines are maintained as isolines by backcrossing to the genetic standard for G. hirsutum, Texas Marker-1 (TM-1) (Kohel et al. 1970). The number of backcrosses to TM-1 varied among the lines.
The hypoaneuploid lines (Table 1) were from the living collection maintained at College Station, TX. Plants of known aneuploidy were used as females to create the initial test hybrids. Aneuploid segregates in F1 and F2 generations were recognized by phenotypic identification. Each type of monosomic and monotelodisomic aneuploid exhibits a characteristic chromosome-specific phenotypic syndrome (Endrizzi et al. 1985). As in the case of the mutant lines, the aneuploids are maintained in a standard isogenic background by crossing to TM-1, with various numbers of prior backcrosses to TM-1.
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Methods |
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Seeds of each population were sown in peat pellets and germinated under spring greenhouse conditions at College Station, TX. While in the greenhouse, seedlings were scored for any expression of the mutant or aneuploid phenotype. At 3 weeks of age, the greenhouse-grown seedlings were transplanted to field plots. Plots consisted of rows with 0.5 m spacing between plants within rows and 1.0 m spacing between rows. This procedure requires fewer seeds, which is important when studying mutants of low productivity, and it allows the observation of phenotypic expression of individual plants free of stress and competition.
Tests were conducted in 1991–1998. Crosses were made with the hypoaneuploid plant as the female parent and the mutant line as the male parent. Transmission of the deficiency through the egg is reduced, and it is essentially zero through the pollen. Selection of the hypoaneuploid F1 hybrid plants was based on morphological features. All the tests reported in Table 1 consist of F2 progeny of haplo F1 plants, except for the cross involving me, in which only segregation in F1 progenies from monosome x mutant are reported
Most simple recessive mutants, such as me and im, are expected to express the recessive phenotype when hemizygous because of the F1 haplo state, which is deficient for the respective homologous chromosome (White and Endrizzi 1965). For simple recessive and dominant mutants associated with a given chromosome deficiency, all F2 progeny from the haplo F1 would be expected to be uniformly mutant in phenotype. Similarly the gl1gl6 glandless phenotype would expectedly occur in the gl1 hemizygous haplo F1 (gl10gl6gl6), because most cottons are Gl1Gl1gl6gl6 and segregation is only at the gl1 locus. All F2 progeny from gl1 hemizygous haplo F1 plants would be glandless stem and glandless boll if both parents were gl6gl6. If the aneuploid was Gl1Gl1Gl60, the association of gl1 could still be determined. The genotype gl1gl1Gl6- has a phenotype with reduced stem glanding, whereas Gl1gl1Gl6- has normal glanding, so the association of gl1 with the deficiency could still be determined in the phenotypes of the haplo F1 (gl10Gl6gl6) and their segregating F2 progeny. The Gl6 allele was introduced to test for its location. The test for association of Gl6 in the F2 progeny from haplo F1 (Gl1gl1Gl60) from the cross of Gl1Gl1gl60 with gl1gl1Gl6Gl6 would be 3:1 glanded versus reduced glands (no glandless) when associated and 15:1 glanded versus glandless with independence. In the case of dominant duplicate factors controlling Raimondal (Ra1Ra2), a locus associated with the deficiency in the F2 would be indicated by a 3:1 (mutant to wild type) versus a 9:7 segregation for independence. The duplicate recessive nonpinking (np1np2) would segregate wild type to mutant 3:1 with association and 15:1 with independence in the F2.
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Results and Discussion |
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TopAbstractIntroductionDescription of Traits and...MethodsResults and DiscussionReferences |
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The cytogenetic tests revealed chromosomal locations of all three of the simply inherited mutants, me, im, and Li2, but none of the six duplicate-factor loci (Table 1). The association of the recessive me with chromosome 9 (H9) was detected by the occurrence of mutant plants in the F1 generation. In the first population grown (n = 23), only one plant was scored as abnormal with the phenotypes of both me and H9. Cytological analysis revealed that it had 24 chromosome pairs and 2 fragments. Meiotic products deficient for arm 9-long that arise from H9 and monotelodisomic 9-short plants are mitotically unstable, and thus H9 plants produce additional chromosome aberrations (Myles and Endrizzi 1989). In the second F1 population (n = 32), two me plants were verified with the H9 phenotype and cytotype (25 chromosome pairs and 1 univalent). None of the F1 produced viable F2 seed. Chromosome 9 bears 5S and 18S–28S rRNA tandem gene clusters, and several cytogenetic lines bear chromosome 9 translocation breakpoints, but me is the first morphological/biochemical marker associated with chromosome 9.
The mutant im was associated with chromosome 3 (H3) when the F2 was uniformly of the im phenotype. The im mutant phenotype was not scored in the F1 because H3 has reduced fiber development that masked and confounded the immature phenotype. Chromosome 3 contains linkage group VI with markers accessory involucre and frego bract (ia and fg).
The dominant Li2 was associated with chromosome 18 (H18) by uniform mutant expression in the F2. The Li2 mutant is completely dominant, so all the F1 plants expressed the mutant phenotype. A prominent feature of H18 is short, dense fibers (Endrizzi et al. 1985), so it might be expected that this chromosome bears genes that control fiber development. Chromosome 18 contains linkage group XVI [open bud (ob) and Yellow pollen-2 (Y2)], but linkage tests of Li2 and ob showed no significant linkage (Narbuth and Kohel 1990).
The duplicate factor mutants, Raimondal, nonpinking, and glandless stem and boll, did not reveal any indication of an association with the aneuploid lines. As an introgressed mutant, Raimondal, was not a productive line, and some populations were too small to be statistically robust, specifically the tests for H2 and Te5lo that involved the A genome, but they were not repeated for this D genome introgressed mutant. In previous tests of gl1, it was found independent of chromosomes 4, 6, 7, and 12 (Kohel 1978).
We have assigned chromosome locations to three mutants and at least narrowed the potential locations for the remaining three that we tested. This information will enhance the mutants value in genetic experiments and in mapping efforts.
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Acknowledgments |
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We thank Dwaine Raska for analysis of the monosome 9 plants.
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Footnotes |
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Corresponding Editor: Prem P. Jauhar
Received March 9, 2001
Accepted December 31, 2001
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References |
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