The Journal of Heredity 2001:92(1)
© 2001 The American Genetic Association 92:43-50
Genetic Analysis of Mutations at Loci Controlling Leaf Form in Cowpea (Vigna unguiculata [L.] Walp.)
From the Department of Crop Protection and Environmental Biology, University of Ibadan, Ibadan, Nigeria.
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Abstract |
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Mutations affecting leaflet number and shape occurred at high frequencies in some cowpea crosses. Mutant plants were nonpetiolate and unifoliolate as opposed to normal plants which were petiolate and trifoliolate. Two types of unifoliolate mutants were distinguishable on the basis of leaf shape which was ovate in one mutant and orbicular in the other. The nonpetiolate and the unifoliolate traits in the two mutants are each controlled by single recessive genes, but the genes controlling the traits in the different mutants were nonallelic. The orbicular leaf shape was also under the control of a single recessive gene. In the F2 and subsequent generations of the cross IBS 2497 x IBS 2625, orbicular-shaped unifoliolate leaf mutants were regularly produced, although the two parents involved in the cross were both trifoliolate. Linkage tests showed that the genes pt-1 for nonpetiolate trait, un-2 for unifoliolate leaf, and orb for orbicular leaf shape in one of the mutants were located on the same chromosome, while the genes pt-3 and un-3 in the other were also linked on a different chromosome. The results of this study provide further evidence indicating the involvement of transposable elements in the mutations observed in the cowpea lines used in this study.
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Introduction |
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In the cowpea (Vigna unguiculata (L.) Walp.) the first pair of simple leaves is succeeded by alternate trifoliolate leaves held on long, grooved petioles. The leaflets are ovate to lanceolate, sometimes hastate, and are subtended by inconspicuous stipels (Summerfield and Roberts 1985). Rawal et al. (1976) reported a nonpetiolate, unifoliolate cowpea leaf mutant plant in which the flowers were also malformed. The unifoliolate trait is controlled by a single recessive gene to which the symbol un was assigned. I (Fawole 1988) observed high frequency mutations at a petiole locus in several cowpea crosses. Data from inheritance study and allelic tests showed that the nonpetiolate trait is controlled by a single recessive gene pt and all the observed mutations occurred at the same locus. The pt gene is nonallelic to a second gene designated pt-2 that controls the expression of the nonpetiolate trait in the unifoliolate mutant plant described by Rawal et al. (1976). The F1 plants obtained by crossing the two different nonpetiolate mutant lines were petiolate and the F2 generation segregated in the complementary epistatic ratio of 9 petiolate:7 nonpetiolate (Fawole 1990).
A major cause of unusually high mutation frequency in living organisms is inherent instability in gene expression resulting from the presence of transposable elements (Fedoroff 1983; McClintock 1951, 1956; Nevers et al. 1986; Peterson 1986, 1995). Integration of a transposable element near or within a gene may inhibit that gene or modify its action. Excision of the element restores gene activity and another gene may become inhibited through the insertion of the transposable element in that gene. Working with originally crossed lines and progenies derived from them, I (Fawole 1988) showed that the high frequency of recessive mutations that occurred at a petiole locus in some cowpea crosses resulted from the unusual behavior of a cowpea cultivar, Ife BPC, when used as the female parent. In addition, in advanced generations of some of the crosses, mutability seemed to be transferred to a new locus controlling the expression of leaflet number, where unifoliolate true leaves were produced instead of the normal trifoliolate leaves. I therefore suggested that transposable elements might be involved in the induction of the high frequency of mutations observed in the cowpea crosses. In this article I present further evidence on the nature of the factors inducing a high frequency of mutations in the cowpea materials used in this study, report the inheritance of some new mutations, and establish the allelic and linkage relationships between the different mutant genes.
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Materials and Methods |
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The origin and leaf characteristics of the cowpea lines used in the inheritance study are listed in Table 1. The morphological characteristics of the germplasm and breeding lines are described by Porter et al. (1974), Fawole (1997), and Fawole and Afolabi (1983). The nature of the original crosses that produced lines G942, G1001, and G1004 was described previously (Fawole 1988). The lines IBS 2497 and IBS 2625 arose in a similar manner from the crosses Ife BPC x TVu 1 and Ife BPC x IT81D-1137, respectively. IBS 2497 was obtained from the F2 generation of the cross as a single nonpetiolate plant with a distinctive nonbranching, indeterminate main stem. The determinate habit was transferred to the line by crossing to TVu 6198, a determinate line. On the other hand, IBS 2625 originated in an F6 row of the Ife BPC x IT81D-1137 cross planted to seeds from a normal petiolate, trifoliolate selection. Two mutants types emerged from the row. One was a nonpetiolate trifoliolate mutant similar to those described earlier, while the other, designated IBS 2625, was petiolate, trifoliolate but with orbicular-shaped leaflets (Figure 1A, left). The flowers of this mutant were malformed, which made natural selfing difficult, but a few of the flowers produced selfed pods. Progeny of selfed orbicular-shaped trifoliolate leaf plants bred true for the orbicular shape. Some nonpetiolate, trifoliolate leaf plants selected from the F6 row later segregated for plants with orbicular-shaped unifoliolate leaves.
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Experimental Crosses |
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In an experiment originally designed to test the allelic relationship between nonpetiolate, trifoliolate plants from different crosses, the following crosses were made: G 942-1 x G1001, G942-1 x G1004, and G942-2 x G1004. Unexpectedly the progenies of the crosses segregated for both normal, trifoliolate, and orbicular-shaped unifoliolate leaf plants, all of which were nonpetiolate. The plants were scored for the normal and mutant phenotypes 6 weeks after planting.
The inheritance of the orbicular-shaped unifoliolate leaf mutant was further investigated by crossing the mutant line to two normal trifoliolate lines, TVu 6198 and IBCR-4. For these crosses, P1, P2, F1, F2, and the two backcross generations were grown on the Teaching and Research Farm of the University of Ibadan in the early planting season of 1986. Six weeks after planting, individual plants were scored for leaflet number, presence or absence of petiole, leaf shape, and flower characteristics. At maturity, between 40 and 60 F2 plants were randomly selected and those that produced sufficient seed were advanced to the F3 generation in the late planting season of 1987. Individual plants in each F3 family were scored as earlier described.
IBS 2625, the orbicular-shaped trifoliolate leaf mutant, was also crossed to two normal lines, Ife BPC and IBS 2497. The parents, F1, and F2 generations of the crosses were grown in the field during the first planting season (April–July) of 1989. The crosses were handled in the same manner as described above. At maturity, 100 normal trifoliolate plants were randomly selected from the F2 population of each cross and were individually harvested. These F2 selections were advanced to the F3 generation in the field during the first planting season of 1990. At the onset of flowering, individual plants of each F3 family were classified for leaf and flower characteristics.
An F3 row of the cross IBS 2497 x IBS 2625 segregated for a new nonpetiolate, unifoliolate mutant phenotype with normal ovate-shaped leaves. The mutant plants were transferred into plastic pots filled with field soil and were taken to a glasshouse. The flowers of the mutant plants were also malformed, but pods were easily obtained from them by hand pollination, and occasional natural selfing also occurred. The five mutant plants were true breeding for the unifoliolate phenotype. The genetic control of the mutant was studied by crossing it to four normal lines—TVu 1509, TVu 6198, Ife BPC, and IBS 2497. The parental lines, F1, and F2 generations were planted on the field during the first planting season of 1992. At flowering, individual plants were scored for leaf and flower traits. Between 40 and 80 F2 plants were randomly selected in each cross at maturity. Those that produced sufficient seed were advanced to the F3 in the second planting season of 1992 and were scored for the traits of interest.
The cross IBS 2497 x IBS 2625 segregated for orbicular-shaped unifoliolate leaf plants in the F2 and F3 generations even though both parents were trifoliolate. The cross was therefore repeated in 1993 and 1996 to ascertain whether the production of the unifoliolate mutant was of regular occurrence in this cross. F2 and F3 generations of the cross were examined on both occasions and the plants were classified on the basis of the leaf traits under consideration.
Allelism Test
In April 1994, seeds of IBS 401, IBS 2497, M-2, and M-3 were grown in a glasshouse, and during flowering period were crossed in all possible combinations. The resulting F1 seeds were planted in the crop garden of the Department of Crop Protection and Environmental Biology, University of Ibadan. Individual F1 plants were scored for leaf and flower traits at the onset of flowering.
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Results |
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Morphology of Normal and Mutant Plants
Two types of unifoliolate plants were found in the experimental populations and these can be distinguished on the basis of leaf shape, which is orbicular in one type and ovate in the other. The wild-type cowpea plants have stipulate, trifoliolate true leaves with long petioles (Figure 2A), while the true leaves of the mutants are stipulate, nonpetiolate, and unifoliolate (Figures 1A [right] and 3A). The orbicular-shaped mutant leaves are further characterized by the leaf apex, which is retuse (Figure 1A) in contrast to the acuminate apex of leaves of wild-type plants. Flowers of normal plants are typically papilionaceous with a large standard petal, two free-wing petals, and two fused-keel petals. The reproductive organs are enclosed within the keel petals. These consist of 10 diadelphous stamens and a bent, bearded, and beaked style (Figure 2B). Plants of the unifoliolate mutants exhibit abnormal flower development. Many flower buds drop off prematurely and when large flowers are produced, all five petals are separate from each other, the stamens are free, and the style is straight (Figures 1C and 3D). Floral abnormality is more severe in the orbicular-shaped leaf mutant, especially with respect to the stigma, which is nonbeaked, smaller in size than normal, round, and covered with hair (Figure 1D, right). As a result of these abnormalities, natural selfing could not occur but selfed pods could be obtained easily by hand pollination in the mutant with ovate leaf and with great difficulty in the other mutant. Both mutants produced fertile pollen which was used successfully in crosses with normal plants as female parents.
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Inheritance of Orbicular Shaped Unifoliolate Leaf
Data on the segregation pattern for leaflet number in crosses between nonpetiolate, trifoliolate selections from different sources are shown in Table 2. Progenies of the four crosses segregated for leaflet number in the ratio 3 trifoliolate:1 unifoliolate, indicating a monohybrid inheritance of this trait. The inheritance of this unifoliolate mutant was further studied by crossing it to two normal lines. The results of the study, presented in Table 3, show that unifoliolate leaf is recessive to trifoliolate leaf. The backcross to the orbicular mutant parent segregated in the ratio 1 ovate:1 orbicular. However, the F2 generation deviated significantly from the 3 trifoliolate:1 unifoliolate ratio in both crosses. Furthermore, fewer F2 lines than expected segregated in the F3 generation, and the pooled F3 data also deviated significantly from the 3:1 ratio. In the F2 and F3 generations of both crosses, there were marked deficiencies in the number of unifoliolate plants and an excess in the number of trifoliolate plants. Examination of the trifoliolate phenotypic class showed that 88 of the 589 trifoliolate plants in the F2 generation of the cross IBCR-4 x M-2 and 35 of the 626 plants in the cross TVu 6198 x M-2 had the orbicular leaf shape of the unifoliolate mutant. No unifoliolate plants with the normal ovate shape were recovered in either the F2 or the F3 generations of the two crosses. Thus the relatively large number of orbicular-shaped trifoliolate plants could not be totally explained on the basis of genetic recombination and most of them probably resulted from reversion from the recessive unifoliolate condition to the wild-type trifoliolate state. Some of the unifoliolate plants also had bifoliolate and trifoliolate leaves (Figure 1B).
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Inheritance of Orbicular Leaf Shape
Leaf shape data from the crosses IBCR-4 x M-2 and TVu 6198 x M-2 were also classified as ovate and orbicular, and tested for goodness-of-fit to the monohybrid ratio. All F1 plants and plants of the backcross to the ovate parent had the ovate leaf shape, while the progenies of the backcross to the orbicular parent and the F2 generation segregated in the 1:1 and 3:1 ovate to orbicular ratios, respectively (Table 4). These results suggest a single-gene control of leaf shape with the ovate shape completely dominant to the orbicular shape. F3 progenies derived from selected F2 plants segregated in the expected 2:1 ratio, while pooled data from segregating F3 families conformed to the 3:1 ratio. Thus the F3 data confirmed the monohybrid inheritance of the orbicular shape.
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Further evidence that the orbicular leaf shape is a separate and independent mutation was provided by the results of crosses between IBS 2625, a petiolate trifoliolate mutant with orbicular leaf shape and two normal lines, Ife BPC and IBS 2497. The F1 plants in the two crosses were ovate in shape, while the F2 data gave a good fit to the 3 trifoliolate ovate:1 trifoliolate orbicular ratio. The F3 data supported the monohybrid inheritance of leaf shape in the two crosses (Table 5). The symbol orb is assigned to the recessive gene which in the homozygous condition determines orbicular leaf shape.
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Inheritance of Unifoliolate Leaf Mutant with Ovate Leaf Shape
The F1 generation of crosses between M-3, the unifoliolate mutant with ovate leaf shape, and normal trifoliolate lines were all phenotypically trifoliolate. In the three crosses studied, the F2 generation segregated in the ratio 3 trifoliolate:1 unifoliolate (Table 6). However, several unifoliolate plants also had bifoliolate and trifoliolate leaves in addition to unifoliolate leaves (Figure 3B). F3 families derived from selected trifoliolate F2 plants gave a good fit to the expected 2 segregating:1 nonsegregating family ratio and the pooled F3 data did not deviate significantly from the 3 trifoliolate:1 unifoliolate ratio.
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The Nonpetiolate Trait
The inheritance of the nonpetiolate trait in mutant line M-3 was investigated in three crosses (Table 7). All F1 plants of crosses between normal petiolate lines and the mutant were petiolate, while the F2 generation segregated in the ratio 3 petiolate:1 nonpetiolate. The results thus suggest that the petiolate trait is determined by a single locus with the petiolate condition completely dominant to the nonpetiolate trait. The F3 data supported the monohybrid hypothesis except in the cross M-3 x TVu 1509, where the pooled F3 data deviated significantly from the 3:1 ratio.
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Recurrent Mutations at the Unifoliolate Locus
The F2 and F3 generations of the cross IBS 2497 x IBS 2625 segregated for plants with unifoliolate leaf, although both parents in the cross were trifoliolate. The same cross was repeated in 1993 and 1996 to determine whether the occurrence of unifoliolate plants was a chance event or a regular feature of the cross. The results are presented in Table 8. In all cases, the F2 and F3 generations segregated for orbicular-shaped unifoliolate plants, but the data did not give a good fit to the 3 trifoliolate:1 unifoliolate monohybrid ratio. However, when the trifoliolate plants with orbicular leaf shape were separated from normal trifoliolate plants and treated as a different phenotypic class, the data gave a good fit to the dihybrid-dominant epistatic ratio of 12:3:1, except in the F2 generation scored in 1989 (Table 8), suggesting an interaction between two independently segregating factors.
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Allelism Test
All of the lines used in the allelism test were nonpetiolate and two of them, M-2 and M-3, were also unifoliolate. Progenies of three crosses—IBS 401 x IBS 2497, IBS 401 x M-2, and IBS 2497 x M-2—were nonpetiolate, which indicates that the nonpetiolate condition in the parents involved in the crosses is controlled by allelic genes.
The three other crosses, IBS 401 x M-3, IBS 2497 x M-3, and M-2 x M-3, produced progenies with petiolate leaves. Thus the results suggest that two different genes with complementary action control the expression of the petiolate trait in these crosses. The symbol pt-3 is assigned to the recessive gene in the mutant parental line, M-3, which in the homozygous condition determines the nonpetiolate trait.
In addition to being petiolate, the F1 progeny of the cross between the two unifoliolate lines were also trifoliolate, suggesting that the genes controlling leaflet number in the two mutants are different and complementary to each other. The symbol un-3 is assigned to the gene in the unifoliolate mutant with ovate leaf shape.
Linkage Relationships
The linkage relationships of the different genes were analyzed by using Linkage-1, a computer program for the detection and analysis of genetic linkage (Suiter et al. 1983). The results, presented in Table 9, show that of the five F2 linkage tests between gene pairs, two suggested independence while three indicated linkage. The loci Pt-1 and Un-2 are linked with a map distance of 19.29, while Pt-1 is also linked with the locus Orb with a distance of 30.18. Thus Pt-1, Un-2, and Orb are located on the same chromosome. The distance between Un-2 and Orb is 10.89. The genes Pt-3 and Un-3 are both located on the same chromosome and have a map distance of 3.77. The genes Pt-3 and Un-3 showed independence with respect to the gene Bpd, which controls peduncle type.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsExperimental CrossesResultsDiscussionReferences |
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Mutations occurred in some selections of the cowpea crosses used in this study without any apparent cause other than the nature of their hybrid origin. When nonpetiolate, trifoliolate plants from different sources were crossed with each other, unifoliolate plants appeared in the progenies in the ratio 3 trifoliolate:1 unifoliolate, suggesting that the two parents in each cross were heterozygous for the unifoliolate trait. However, when the unifoliolate mutant was crossed to normal trifoliolate plants, the F2 and F3 generations deviated significantly from the expected monohybrid ratio. An examination of the plants in the two phenotypic classes showed an excess of trifoliolate plants and a marked deficiency of unifoliolate plants. Furthermore, the trifoliolate plants that were in excess were those that had the mutant orbicular leaf shape. The orbicular leaf shape results from a separate mutation from that affecting leaflet number and is controlled by a single recessive gene. The joint segregation data for the two traits, however, indicated that they are closely linked. If recombination occurred between the two loci, both orbicular-shaped trifoliolate and ovate-shaped unifoliolate recombinant plants are expected in the F2 generation. On the contrary, only orbicular-shaped trifoliolate recombinant plants were obtained. The relatively large number of these plants could not be explained solely on the basis of recombination, therefore reversion from the recessive unifoliolate to the dominant trifoliolate form was probably responsible for the occurrence of most of these plants. Support for the reversion hypothesis is furnished by the fact that several of the plants that were classified as unifoliolate also had some leaves that were trifoliolate, bifoliolate, and even at various stages of incomplete division toward trifoliolate.
In the orbicular-shaped unifoliolate mutant, three closely linked genes have been identified and genetically characterized. These are the genes that control the unifoliolate trait, the nonpetiolate leaf, and the orbicular leaf shape. Rawal et al. (1976) described a unifoliolate mutant in a cowpea line and assigned the symbol un to the recessive gene controlling the trait. The symbol un-2 is proposed for the gene that controls the unifoliolate trait in the mutant with orbicular leaf shape. On the other hand, two linked but different genes control the unifoliolate and the nonpetiolate traits in the mutant with the ovate leaf shape. In a previous report, Fawole (1988) assigned the symbol pt to the petiole locus. Subsequently the gene was redesignated pt-1 (Fawole 1990) to distinguish it from the gene controlling the nonpetiolate trait in a mutant plant described by Rawal et al. (1976). The pt-1 gene is nonallelic to the gene controlling the nonpetiolate trait in M-3, the unifoliolate mutant with ovate leaf shape used in this study, and the symbol pt-3 is therefore suggested for the petiole gene in M-3.
The appearance of a new nonpetiolate unifoliolate mutant in the F3 generation of the cross IBS 2497 x IBS 2625 suggests the transfer of mutability to a new locus. Unlike the orbicular-shaped unifoliolate mutant, the inheritance data of this new unifoliolate leaf mutant showed no deviation from the monohybrid ratio. However, some of the unifoliolate plants in the F2 and backcross generations produced leaves that were bioliolate, trifoliolate, or at various stages of incomplete division toward trifoliolate in addition to unifoliolate leaves. In both unifoliolate mutants, M-2 and M-3, the genes controlling the nonpetiolate trait are separable by recombination from those controlling the unifoliolate leaf, although the ovate-shaped unifoliolate mutant indicated closer linkage with the nonpetiolate trait than the other mutant. Similarly the orbicular leaf shape gene is separable from the nonpetiolate trait. However, no recombination was observed between the genes controlling leaf shape and leaflet number. This is probably due to very close linkage of these genes.
Instances of single gene mutations resulting in unifoliolate true leaves have been reported in some leguminous species. Lamprecht (1935) described a highly sterile unifoliolate mutant in Phaseolus vulgaris which is controlled by a single recessive gene, while Garrido et al. (1991) reported another unifoliolate mutant in the same species, encoded by a single dominant gene with some deleteriousness in unifoliolate homozygotes. In Pisum sativum, Hofer et al. (1997) observed that the unifoliolate gene has a pleiotropic effect on flower development and therefore concluded that leaves and flowers share regulatory processes in their morphogenesis that may be useful in the study of plant development. The mutations that are reported in this study affect closely linked genes that control leaf form traits in cowpea. However, abnormal flower traits of both unifoliolate mutants are inherited together with unifoliolate leaf, as if flower malformation results from the pleiotropic effect of the unifoliolate genes. Results from other crosses, however, suggest that the genes controlling flower form are distinct from but tightly linked to those determining leaflet number (Fawole I, unpublished data).
The origin and nature of the nonpetiolate unifoliolate mutations are similar to those reported previously in certain cowpea crosses (Fawole 1988, 1997). In those crosses the cultivar Ife BPC, when used as the female parent, facilitated the occurrence of high frequency mutations at certain loci. Further evidence of this unusual behavior is furnished by the advanced generation progeny of the cross Ife BPC x IT81D–1137. In addition, it appears that some lines derived from such crosses can inherit the "mutator" ability in a more potent manner than Ife BPC. For example, the orbicular-shaped unifoliolate mutation can be induced at will by making the IBS 2497 (female parent) x IBS 2625 (male parent) cross and searching for the mutant in the F2 and F3 generations. Transposable element activity had been implicated in the occurrence of the unusually high mutation frequencies observed in segregating populations of crosses involving Ife BPC as the female parent (Fawole 1988, 1997).
The behavior of the nonpetiolate, unifoliolate mutations described in this article also exhibit some features attributable to the action of transposable elements. First, the insertion of a transposable element at a locus may inhibit the expression of that gene and confer on it the property of high mutability. The excision of the element from the locus restores the activity of the gene and the reintegration of the element at another locus may result in instability at the new locus (Fedoroff 1983; McClintock 1956; Nevers et al. 1986; Peterson 1968). The nonpetiolate unifoliolate and the orbicular leaf shape mutants probably resulted from such insertion and excision events.
Second, mutations mediated by the action of transposable elements often exhibit unstable expression and revert to a phenotypically wild-type condition in somatic and/or germinal tissue. Frequent reversions to wild type in germinal tissue may lead to distorted Mendelian ratios in crosses involving a mutable allele (Nevers et al. 1986). Data on the inheritance of the orbicular-shaped unifoliolate mutant showed that when stable unifoliolate plants were crossed to normal lines, there was significant deviation from the expected monohybrid ratio. The deviation was caused by an excess of wild-type trifoliolate individuals that could not be explained on the basis of genetic recombination and are probably due to reversion of the recessive unifoliolate gene to the wild-type trifoliolate condition. Evidence of reversion was also provided by several unifoliolate plants that showed various degrees of reversion from the unifoliolate to the trifoliolate state.
Third, crossing is often necessary to initiate mutability. For example, in the nonautonomous two-element system of maize, the gene influenced by a transposable element remains stable until a regulatory element is introduced into the system by crossing. Only then does a transposable element induce mutation which may be to a recognizable change in phenotype (Dooner 1983; McClintock 1951, 1953, 1956; Peterson 1986, 1995). In the cowpea lines used in this study, mutability normally occurred only when Ife BPC and IBS 2497 were used as female parents in crosses to certain selected lines. This suggests a two-component system in which factors in the genome of Ife BPC and IBS 2497 responded to signals from the lines used as male parents.
The involvement of two independently segregating factors is indicated in the IBS 2497 x IBS 2625 cross, which not only repeatedly produced a trait not present in either of the two parents but also consistently showed interaction between factors that are located on the chromosomes of the two parental lines. These factors segregated in the dominant epistatic ratio of 12:3:1 in the F2 and F3 generations.
The phenotypic changes reported in this study represent only a portion of the total variation that has been generated in crosses between the mutable and normal lines. New mutations have been identified at loci controlling stem growth habit (Etta 1995), flower form and anthocyanin pigmentation on pods and flowers (Fawole 1998; Oluwatosin 1997), and foliage color (Fawole 1997). Some of these new mutants have been studied while others are yet to be genetically characterized.
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Acknowledgments |
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The author thanks the University of Ibadan for providing facilities for the research reported in this article.
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Footnotes |
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Address correspondence to I. Fawole % Dr. N. Q. Ng, IITA, % L. W. Lambourn & Co., Carolyn House, 26 Dingwall Road, Croydon CR9 3EE, England, or e-mail: library{at}kdl.ui.edu.ng.
Corresponding Editor: Susan Gabay-Laughnan
Received March 20, 2000
Accepted August 31, 2000
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