Genetics of a New Male-Sterility Locus in Pigeonpea (Cajanus cajan [L.] Millsp.)

Abstract

A natural male-sterile mutant was found in the population of a short-duration pigeonpea (Cajanus cajan[L.] Millsp.) cultivar ICPL 85010. This mutant is characterized by light yellow anthers of reduced size that are devoid of pollen grains. This mutant was crossed with two pigeonpea cultivars to study its inheritance. The F₁, F₂, and test cross data of the two crosses suggested that this male sterility trait is genetic in origin and is controlled by a single recessivegene. The F₁ (mutant × ICPL 85010) plants were crossed with translucent (ms₁) and arrowhead type (ms₂) genetic male steriles reported earlier to study their allelic relationships. Segregation in the three-way cross F₁ and F₂ populations revealed that the mutant male-sterile gene was nonallelic to ms₁ and ms₂ loci and it is designated ms₃. The new male sterility sources in pigeonpea will help in producing high-yielding hybrids and populations in diverse phenological groups.

Pigeonpea is a short-lived perennial legume shrub. It is mainly cultivated for its dry seeds and green vegetable in dry areas of the tropics and subtropics. In comparison to other food legumes, the pigeonpea plants differ grossly especially with respect to their pollination behavior. The bright colors of pigeonpea flowers attract a number of insects, which affect cross-fertilization with an average natural outcrossing of up to 20% (Saxena et al. 1990). This phenomenon is primarily responsible for the deterioration of varietal purity in this crop. Pigeonpea breeders, however, are using natural outcrossing for the genetic improvement of populations (Byth et al. 1981) and breeding hybrids (Saxena et al. 1996). So far two sources of male sterility have been identified for use in breeding programs. They include a translucent anther type (Reddy et al. 1978) and another with arrowhead-shaped anthers (Saxena et al. 1983). These two male steriles are morphologically distinct and controlled by nonallelic recessive alleles at a single locus, designated as ms₁ and ms₂ respectively.

In the 1997 rainy season, a natural mutant plant completely devoid of pollen grains was identified in the population of the short-duration determinate cultivar ICPL 85010. A close examination of its flowers revealed that the anthers of this mutant were morphologically different from the male-sterility sources reported earlier and were characterized by light yellow color and reduced size. The objective of this article is to report the inheritance of the new male-sterile mutant and its allelic relationship with translucent (ms₁) and arrowhead-shaped (ms₂) male-sterile sources.

Materials and Methods

The mutant plant was crossed with eight randomly selected plants from the same plot for its maintenance, but of these, only one crossed progeny segregated for male sterility. To study the genetics of this trait, the male-sterile segregants of this progeny were crossed with cultivars ICPL 88039 and ICPL 85010. The F₁ plants were used for producing F₂ and testcross populations. To study the genetic relationship of the mutant with ms₁ and ms₂ loci, the F₁ (mutant × ICPL 85010) plants were crossed with other male-sterile lines—MS Prabhat carrying ms₁ allele and MS QPL2 carrying ms₂ allele. The triple cross F₁ plants were selfed for generation advance, and their F₂ progenies were grown in 1999 to study the segregation pattern of different male-sterility loci. The data from different crosses were subjected to chi-squared analysis.

Results and Discussion

Inheritance of the Male-Sterile Mutant

In the F₁ generation all the plants in both the single crosses were fertile. Of the 231 plants grown in the F₂ generation of the mutant × ICPL 85010 cross, 171 were fertile and 60 sterile (Table 1), fitting a 3:1 ratio (P < .8). In the other cross (mutant × ICPL 88039), 85 F₂ plants were fertile and 38 sterile, showing a good fit to a 3:1 ratio (P < .5). The pooled data from the two crosses also exhibited a good fit to a 3:1 ratio (P < .8). The testcross populations of the two crosses and their pooled data fit a 1:1 ratio (P < .2), confirming that the mutant male-sterile trait was controlled by a single locus with recessive alleles.>

Table 1.

Segregation for male sterility in F₂ and testcross progenies of two crosses involving male-sterile mutants and cultivars

	Cross
	Mutant × ICPL 85010		Mutant × ICPL 88039		Pooled data
Generation	Fertile	Sterile	Fertile	Sterile	Fertile	Sterile
F1	27	0	11	0	38	0
F2	171	60	85	38	256	98
Probability (3:1)	.7–.8		.3–.5		.7–.8
Test cross F1	70	59	36	28	106	87
Probability (1:1)	.1–0.2		.3		.1–.2

Allelic Relationship with ms₁ and ms₂

Genes

In the three-way cross (MS Prabhat × F₁ [mutant × ICPL 85010]), involving the ms₁ gene and the new mutant, all the F₁ plants were fertile, indicating the nonallelic nature of the two male-sterile systems. F₂ progenies of nine three-way cross F₁ plants were studied further to determine the segregation patterns of the two different male-sterile types. Of these, four progenies segregated for fertile and mutant male-sterile types. The pooled segregation within this group across the progenies (Table 2) revealed a good fit to a 3 fertile: 1 sterile ratio (P < .3). The remaining five three-way F₂ progenies segregated, besides fertility, for both the translucent-type (ms₁) as well as the mutant-type male steriles in a 9:3:4 ratio (P < .7). These results suggest that the two male-sterile genes segregated independently and the double recessive resembles the phenotype of a mutant male sterile. The proportion of progenies segregating for only one type of male sterility and those segregating for two types of male sterility fits a 1:1 ratio (P < .8).

Table 2.

Segregation for male sterility in two three-way cross F₁ and F₂ populations

			No. of plants
Cross	Generation	No. of progeny	Fertile	Mutantsterile	ms2 sterile	ms1 sterile	Probability
MS Prabhat (ms1) × F1a	F1	—	23	0	0	0	—
	F2	4	71[3]	31[1]	0	0	.2–.3
		5	131[9]	51[4]	0	39[3]	.5–.7
	Probability (1:1)	.7–.8	—	—	—	—	—
MS QPL (ms2) × F1a	F1	—	13	0	0	0	—
	F2	9	87[3]	38[1]	0	0	.1–.2
		12	85[9]	32[3]	44[4]	0	.5–.7
	Probability (1:1)	.5–.7

^aMutant × ICPL 85010.

[] Expected phenotypic proportion.

In the second three-way cross, where the F₁ (mutant × ICPL 85010) plants were crossed to the arrowhead anther-type male-sterile plants, all the three-way cross F₁ plants were fertile, indicating nonallelic control of these two male-sterility systems. Of the 21 F₂ progenies sown, 9 segregated for fertility and mutant male-sterile types and 12 progenies segregated for fertility and the two male-sterile types. This distribution of the progenies fit to the expected ratio of 1:1 (P < .7). In the first group of progenies the pooled segregation for the fertile and mutant sterile plants fit a 3:1 ratio (P < .2). In the other group of progenies, the pooled data showed that the segregation for fertile, mutant male sterile, and arrowhead anther-type (ms₂) male sterile followed the pattern of a 9:3:4 ratio with a good fit (P < .7). These observations also suggested that the loci for these two male-sterile systems segregated independently and the double-recessive plants resembled arrowhead anther-type male steriles.

The inheritance studies show that the mutant male sterile identified in the population of ICPL 85010 is a new source of male sterility. The expression of this trait is controlled by a single locus with recessive alleles and it is nonallelic to the earlier reported ms₁ and ms₂ genes. For this new male-sterile mutant, a gene symbol ms₃ is designated.

Reddy et al. (1978) reported that in the translucent (ms₁) type of male sterile, the anther shape and size develop normally and the male sterility is caused by nonseparation of tetrads due to persistent tapetum. In a similar study, Dundas et al. (1981) found that the arrowhead (ms₂) type of male sterility is conditioned by the breakdown of microsporogenesis at an early tetrad stage. Therefore, unlike ms₁, the shape of anthers in ms₂ is different and resembles an arrowhead. Saxena et al. (1983), while studying the allelic relationship between ms₁ and ms₂ genes, reported that in the double-recessive ms₁ms₁ms₂ms₂ genotype of the ms₂ gene expresses earlier than the ms₁ gene, and therefore its phenotype resembles that of the ms₂ male sterile. In the present case, the studies on microsporogenesis of the new mutant were not performed, but the segregation patterns observed in the three-way cross F₂ progenies suggested that in the ms₁ms₁ms₃ms₃ genotype, the ms₃ gene acts earlier than ms₁, resulting in a mutant-type phenotype. On the other hand, in the ms₂ms₂ms₃ms₃ genotype, the ms₂ gene is activated earlier than ms₃ and thus produces a relatively higher frequency of arrowhead-type male steriles (Table 2). Therefore, considering all three genes together, it is postulated that in the ms₂ms₂ genotype the breakdown of microsporogenesis is at the earliest stage, followed by ms₃ms₃ and ms₁ms₁

This new source of male sterility has enriched the genetic resources of pigeonpea and will help in diversifying the genetic base of male steriles in breeding high-yielding hybrid cultivars and populations. Since it is an inherited trait, it can also be transferred to various agronomically superior backgrounds to develop heterotic hybrids in different phenological groups.

Corresponding Editor: Prem P. Jauharp