Abstract: Eight parents in eight cross combinations were studied for heterosis in F1 a nd inbreeding depression in F2 populations for seedcotton yield, number of bolls per plant, ginning outturn percent and staple length. Seedcotton yield and number of bolls expressed considerable amount of heterosis, however, hybrids showing higher magnitudes of heterosis were generally associated with higher amount of inbreeding depression suggesting dominant genes functioning for these traits. Small amount of heterosis, lower magnitude of inbreeding depression for lint percent and staple length indicated that additive genes were responsible for the expressions of these traits. Hybrid vigor recorded in F1 in respect of seedcotton yield per plant ranged from 14.9 to 88.7%, whereas in bolls per plant, ginning outturn and staple length, the range was 13.5 to 91.6, -0.57 to 4.29 and -5.92 to 5.23% respectively.
Introduction
Crosses between inbred lines show vigor and productiveness in F1 generation (Shull, 1908), but with increasing homozygosity due to selfing, vigor and productiveness reduces by 50% in each generation because of inbreeding depression (Falconer, 1989). Heterosis and inbreeding depression are complementary to each other and the two phenomenons are usually observed in the same studies. Thus, the character, which shows high heterosis due to dominant allelic factors proportionally show high inbreeding depression because of fixation of allelic genes with, increased homozygosity. Gunaseelain and Krishna Swami (1988), Vyahalkar et al. (1984) and Bhatade (1984) also reported that high heterosis was generally associated with high inbreeding depression.
In polyploids like cotton, it may not hold true that complex traits show less depression than the simpler traits. Aycock and Wilsie (1968) found that in auto-tetraploid alfalfa, the yield decreased twice as much as predicted. This response according to them was attributed to a decrease in favorable interactions among multiple alleles due to inbreeding and abnormal segregation at meiosis because of higher ploidy. Gupta and Singh (1987) and Katageri et al. (1992) recorded 81.85, 12.2, 69.4 and 5.4 percent inbreeding depression for seedcotton yield, boll weight, number of boils and staple length respectively. Present studies were therefore carried out to provide information on heterosis, type of gene action and relative inbreeding depression for simpler and more complex traits of upland cotton.
Materials and Methods
Three cotton leaf curl virus disease resistant varieties viz., CIM-435, CIM-443 and CIM-448, five exotic glandless varieties F-280 gl, F-281 gl, Gregg-25V, Glandless Rex and Gossypol free seed were crossed during 1997. The F1 seed from 8 cross combinations was grown in 1998 to raise F2 population and fresh crosses were also attempted to compare F1s and F2s simultaneously. In 1999, F1 and their F2 populations combined with parents were grown in a Randomized Complete Block design with four replications. Two rows of F1s and three rows of F2s and parents were planted in each replication. Twenty-five plants from each replication, totaling 100 plants of each parent, F1 and F2 were selected at random and treated as index plants for recording the data. The standard analysis of variance method developed by Gomez and Games (1984) was adopted. Hybrid vigor compared with respective high parents was calculated for F1 and F2 hybrids as under:
where F1 and F2 were the mean values of first and second filial generations and HP being the high parent value for each economic trait.
The inbreeding depression in F2 population was determined as percent decrease (-) of F2 against their respective F1 hybrids as under:
The observations were recorded on number of bolls per plant, seedcotton yield per plant (gm), lint percent and staple length (mm).
Results and Discussion
The mean performance of F1 hybrids, F2 population and their parents has been depicted in Table 1. The mean squares from the analyses of variance are presented in Table 2 that demonstrates significant differences among F1, F2 and the parents for all the four traits under investigation. In the mean performance, all the 8 F1 hybrids set more bolls, gave higher seedcotton yield, ginned better and gave longer fibers than their respective parents, whereas majority but not all the F2s were superior to their respective parents for these traits. The data of heterosis in the F1 and F2 and the percentage of inbreeding depression in F2 for all the traits are depicted in Table 3. For number of bolls per plant the average heterosis in F1 was 56.9 percent, however the maximum heterosis of 91.6 percent was shown by CIM-435 x Gregg-25V followed by CIM-443 x Gregg-25V (87.5%). F2 population although manifested 3.96% average heterosis, nevertheless, two, out of eight combinations suffered from considerable amount of inbreeding depression.
| Table 1: | Mean performance of Parents, F1 and F2 hybrids for four economic characters |
| Table 2: | Mean squares from analyses of variance for four economic characters |
| ** = Significant at 5% level | |
| Table 3: | Heterosis in F1 hybrids and inbreeding depression in F2 population |
| Table 4: | Heterosis in F1 hybrids and inbreeding depression in F2 population |
Minimum and maximum depressions were noted in CIM-448 x Glandless Rex and CIM-448 x Gossypol free seed combinations, respectively, On an average, the inbreeding depression in F2 was about halfway smaller to that of heterosis expressed in the F1. These results coincide with the theoretical assumptions that vigor and productiveness in F2 reduces by 50 percent in each selfing generation. It is also interesting to note that combination CIM-448 x Gossypol free seed manifested third maximum heterosis for number of bolls also suffered from maximum inbreeding depression. Our results that high heterosis was generally associated with high inbreeding depression are in conformity with those of Gunaseelain and Krishna Swami (1988) and Wang and Pan (1991).
The association of heterosis and inbreeding depression suggested that dominant and over-dominant genes are responsible for number of bolls. Katageri et al. (992) also recorded 34.5 to 69.4 percent inbreeding depression for this trait. Majority of the F1s has shown above 50 percent heterosis suggesting that these hybrids can be useful for hybrid cotton development. High inbreeding depression for number of bolls in F2 could be explained by abnormal segregation at meiosis due to higher ploidy and dissociation of favorable dominant factors due to selfing (Table 4).
Almost same trend of heterosis and inbreeding depression was noted for seedcotton yield also. Average heterosis for this trait recorded was 56.2% whereas the range was 14.9 to 88.7%. Highest heterosis (88.7%) for seedcotton yield was manifested by combination CIM-435 x Gregg-25V followed by CIM-443 x Gregg-25V (86.1%) and CIM-448 x Gossypol free seed (76.3%). The results suggest that parent Gregg-25V may be rated as good general combiner and may be used in hybrid cotton production successfully. Maximum inbreeding depression (-40.9%) in F2 was observed in CIM-435 x F-281 gl combination followed by CIM-448 x Gossypol free seed (-37.5%) for seedcotton yield per plant. The results for this trait have also proved that those combinations, which manifested maximum heterosis also suffered from maximum inbreeding depression. These results confirm the previous studies by Gunaseelain and Krishna Swami (1988) and Wang and Pan (1991).
As regards ginning outturn percent, only two combinations recorded negative heterosis in F1 generation where the maximum positive heterosis of 4.29% was manifested by combination CIM-443 x Gregg-25V. In F2 generation only the same combination gave positive heterosis otherwise the rest of the combinations manifested negative heterosis. For staple length, three combinations out of eight, recorded negative heterosis in F1 while all the combinations gave negative heterosis in F2 generation.