Abstract
The study was carried out for the characterization of 38 onion germplasm using
morphological and molecular markers, genetic diversity study and combining
ability studies of diallel crosses involving 5 genotypes in onion during the period
from 2010 to 2013 at the Spices Research Center, Bangladesh Agricultural
Research Institute (BARI), Bogra and Bangladesh Agricultural University (BAU),
Mymensingh. The germplasm exhibited wide range of variation on different
morphological and qualitative characters, yield and yield components. Among 38
germplasm of onion AC0245 was the best in respect of individual bulb weight and
yield. In all respect of traits, genotypic coefficient of variation (GCV) was always
smaller than phenotypic coefficient of variation (PCV). Narrow differences
between GCV and PCV and high heritability coupled with high genetic advance
confirmed least environmental effects on individual bulb weight and yield per
hectare. This offered better scope for selection. Based on multivariate analysis, the
germplasm were grouped into 6 clusters. There was no relationship between
genetic divergence and geographical distribution of the germplasm. Yield per
hectare had the maximum contribution towards total divergence. Genetic
diversity of 38 germplasm was analyzed using variation at 34 RAPD loci, of which
91.89% were polymorphic. The highest pair wise genetic distance was 1.046 in
between AC0283 and ACO296. The mean gene diversity among all the germplasm
was 1.54. The UPGMA dendrogram, based on Nei`s genetic diversity, demonstrated
6 clusters. Clustering of the germplasm based on morphological characters did
not match with the clustering obtained from RAPD analysis. Studies on
interrelationships among yield and yield components revealed that genetic
correlation coefficient was higher than corresponding phenotypic ones for all the
studied characters, thereby, establishing strong inherent relationships among
them. Path coefficient analysis showed that the bulb diameter had maximum
direct effect on bulb yield followed by neck diameter at after harvest genotypic
level. Success of crossing varied from 44-70% during diallel crosses between
selected five genotypes. Out of 10 cross combinations, only one cross showed
significant positive hybrid vigor in respect of almost all the important characters.
The highest better parent heterosis (25.84) was found in P₁×P4 cross combination
in respect of bulb yield. The mean square values for combining ability analysis
showed that both general combining ability (GCA) and specific combining ability
(SCA) were highly significant for all the characters studied, indicating that
additive and non-additive genetic components were responsible for expression of
those parents. Parents P1 and P4 were good general combiners for almost all the
important characters. Estimates on specific combining ability (SCA) and heterosis
in F1 generation showed that there was a P1 × P4 cross having significantly
positive and expected negative effects for different important characters of onion,
indicating the possibilities of exploiting these crosses or use of these crosses in the
development of hybrid varieties.