Abstract:

Yam bean (Pachyrhizus species) is a legume root crop newly introduced in East and Central
Africa for food production. The crop has nutritious storage roots rich in protein, iron, zinc and
vitamins in addition to wide cco-geographical adaptation and biological nitrogen fixation.
However, the acceptance, adoption and utilization of yam bean is constrained by low storage root
dry matter content (RDM) and yield yet high RDM is preferred for consumption and industrial
applications among staple root crops in Africa. Thus genetic studies to improve RDM, yield and
associated traits were needed as a basis for designing yam bean improvement programs. This
study presents the first comprehensive results on genetic characterization, gene action,
inheritance and response to selection for yield, RDM and nutritional quality traits in the yam
beans. The objectives of this study were to: i) estimate genetic variability for RDM, yield and
quality traits among the introduced yam bean germplasm in Uganda, ii) determine combining
abilities and gene action for inheritance of RDM and associated traits in Fi and F2 yam bean
populations and iii) assess response to selection for RDM and associated traits in selected F3 yam
bean populations.
Four trials were conducted for genetic characterization of RDM, yield and associated traits in 26
accessions of the three cultivated yam bean species at two locations over two seasons as a basis
for parental selection. Then hybridisation was done by crossing three high RDM P. tuberosus -
chuin cultivar and six low RDM accessions of P. erosus and P. ahipa using North Carolina II
mating design (NCD II) and to generate Fi populations. Field evaluations were done for parental
lines, Fi plants, F2 segregating populations, and 83 F3 families selected at 10% on the basis of
high RDM. A randomised complete block design (RCBD) was used in all field experiments with
spacing of 0.3m between plants and Im between ridges. Data was collected on storage root fresh
yield (RFY), storage root dry yield (RDY), vine yield (VNY), fresh biomass yield (FBY),
harvest index (HI), protein (PRO), starch (STA), iron (Fe), zinc (Zn), potassium (K) and
phosphorus (P) content of storage roots. The Near-infrared reflectance spectroscopy (NIRS) was
used to determine the nutritional quality of storage root flour samples. Statistical and genetic
analyses were done through analysis of variance, variance component analyses, combining
ability and correlational analyses.
The results from genetic characterization of the 26 yam bean accessions revealed significant
(p<0.01) genetic variation (g2g) for RFY, RDM, RDY, VNY, FBY, STA, iron (Fe) and high
broad sense heritabilities ranging from 58.4% to 83.6%. Similarly, the phenotypic coefficients of
variation were high for RFY (66%), RDY (53.3%), VNY (60.5%), and FBY (59%), but medium
for RDM (22.6%), STA (15.1%) and Fe (21.3%). Strong positive correlations were observed
between RFY and both RDY (r = 0.926) and FBY (r = 0.962), besides RDY and FBY (r = 0.899)
but low to moderate correlations amongst quality traits. Conversely, the results of study two on
combining abilities and gene action revealed significant (p<0.01) mean squares of crosses for
RDM, STA, RFY, RDY and FBY in Fi and all traits in F2. The GCA mean squares for females
(GCAf) and males (GCA,n) were also significant (p<0.01) for RFY, RDY, FBY, STA, PRO and
RDM, RDY, STA respectively while SCA mean squares were only significant (p<0.05) for
RDM, RFY and STA in F, generation. In F2 generation, GCAf were significant (p<0.01) for all
traits except RDM and VNY but GCA,„ were significant for RDM, RFY, RDY and FBY.
Likewise, the SCA mean squares were significant (P<0.01) for RDM, RFY, HI, FBY and STA
suggesting both additive and non-additive gene action.
The narrow sense heritability and baker’s ratios were moderately high (>0.5) for all the traits
except VNY and PRO in Fj and VNY in F2 indicating the predominance of additive genetic
effects and suggesting that performance of progenies could be reliably predicted from parents for
most traits. The GCA effects were positive and significant (P<0.01) for RDM and STA among
parents 209014 and 209018 in Fj and F2 respectively and for RDM, RFY, RDY and FBY of
parent 209016 in F2. While the SCA effects were positive and significant (p<0.01) for RDM in
crosses; 209022 x 209014 and 209019 x 209013 in F, and 209016 x 209013 and 209031 x
209014 in Ft. These parents and crosses are promising for improvement of RDM RFY, RDY,
FBY, and STA in yam beans. The results of study three from analysis of variance revealed
significant genetic variation (p<0.001) among the 83 F3 selected families for all the eight traits.
Additive genetic variances (o2a) were high and above unity for all the traits except VNY and
PRO. Moderately high GCV values were observed for STA (10.28), RDM (13.42), RFY (18.8),
RDY (23.71), FBY (19.91) and VNY (25.25). High narrow sense heritability (0.53 - 0.74%)
and percentage response to selection (15.5 - 33.12%) were obtained for RDM, RFY, RDY,
VNY, FBY and STA indicating high potential for genetic improvement of these traits in the
yam bean germplasm in Uganda.
In conclusion, the three studies demonstrated that the yam bean germplasm in Uganda possesses
considerable genetic variation that should allow genetic gains for RFY, RDM, RDY, FBY, STA
and perhaps root Fe and PRO contents. The observed genetic variation ( o2G = 9.3%2 ) for RDM
suggested that it is possible to breed for high dry matter yam beans by using low dry matter
accessions without having access to high dry matter P. tuberous-chwms. The inheritance of
RDM, RFY, RDY, FBY and STA was controlled by both additive and non-additive gene effects
but with predominance of additive genetic effects. Lastly, the combination of high genetic
variance, heritability and response to selection obtained in F3 indicated potential to obtain high
genetic gains for RDM, RFY, RDY, VNY, FBY, STA and Fe signifying that a systematic
breeding program utilizing the available yam bean germplasm would develop well adapted, high
yielding and high dry matter, protein and nutrient rich root crop for East and Central Africa.