The F2 data showing the mode of segregation in different F2 families including disomic cross, are presented in Table 2. A good fit to a ratio of 15 fertile: 1 sterile was obtained in the F2's of the disomic cross (control) as well as in the 19 families of the monosomic F2's. In crosses involving chromosomes 4A and 6B expected digenic segregation was not observed. 425 plants out of 576 from the progeny of monosomic 4A F1 and 280 plants out of 381 from the progeny of monosomic 6B F1 showed full fertility, while 151 plants from the progeny of 4A F1 and 101 plants from the progeny of 6B F1 showed partial sterility. Segregation pattern of fertile and sterile deviated significantly from the expected ratio of 15 fertile : 1 sterile which is expected if the chromosomes 4A and 6B are involved in the regulation of fertility trait. Based on these results it is suggested that the p-mst is controlled by two genes located on chromosomes 4A and 6B, respectively.
Discussion
The parents involved in the pedigree of p-mst strain are Selection 212,
a wheat-rye recombinant (derived from the crosses involving monosomic 5B of
cv. Chinese Spring, rye and var. Sonalika) and HD 2009 (an Indian wheat). P-mst
strain is an highly evolved strain in the background of hexaploid wheat (2n=42).
Compared to other major cereals like maize, rice and barley, the number of spontaneously
arisen Mt mutants in wheat is low mainly because of its hexaploid nature. Genic
male-sterility reported by Pugsley and Oram (1959), Lupton and Bingham (1966-1967),
Athwal et al. (1967) and Lemekh et al. (1971) was associated with pollen sterility.
The p-mst under report is genic male-sterility which is caused by the modification
of anthers into ovaries. Central portion of p-mst spike exhibit a greater degree
of anther transformation than the upper and lower parts of the spikes. On an
average 10 to 12% seeds are produced by the p-mst plant. Similar genic male-sterility
was reported by Jan and Qualset (1977), showing high influence of environmental
factors over ms gene action. In the present case the p-mst is least influenced
by environment.
The control of genic male-sterility strain was reported by one recessive gene
(Singh 2002). For inheritance study p-mst was crossed with a fully fertile hexaploid
wheat cv. Kundan. The results of monosomic analysis revealed the involvement
of two recessive genes for controlling male-sterility in p-mst strain. The genes
have been located on chromosomes 4A and 6B. The difference in the results thus
could be due to the variable parents used in the studies. In the inheritance
study (Singh 2002) the p-mst was crossed with a recently released Indian hexaploid
wheat (cv. Kundan) while in the monosomic study cv. Chinese Spring and its aneuploid
lines were utilized. It is, therefore, assumed that for normal anther development
cv. Chinese Spring carries two genes (on 4A and 6B ) while cv. Kundan carries
only one gene (on 4A). The gene located on chromosome 6B in cv. Kundan may be
in recessive form.
Location of gene for mst trait on chromosome 4A confirms the finding of Driscoll
(1975) and Kleijer and Fossati (1976) where ms genes of Pugsley and Probus
mutants were located, on chromosome 4A. Location of another gene for mst trait
on chromosome 6B, is in support of the findings reported by Sears (1954) where
it was reported that in nullisomic X (later changed to chromosome 6B) the stamens
get replaced by pistils leading to male-sterility.