Cassava Breeding II: Phenotypic Correlations through the Different Stages of Selection

Abstract

Breeding cassava relies on a phenotypic recurrent selection that takes advantage of the vegetative propagation of this crop. Successive stages of selection (single row trial–SRT; preliminary yield trial–PYT; advanced yield trial–AYT; and uniform yield trials UYT), gradually reduce the number of genotypes as the plot size, number of replications and locations increase. An important feature of this scheme is that, because of the clonal, reproduction of cassava, the same identical genotypes are evaluated throughout these four successive stages of selection. For this study data, from 14 years (more than 30,000 data points) of evaluation in a sub-humid tropical environment was consolidated for a meta-analysis. Correlation coefficients for fresh root yield (FRY), dry matter content (DMC), harvest index (HIN), and plant type score (PTS) along the different stages of selection were estimated. DMC and PTS measured in different trials showed the highest correlation coefficients, indicating a relatively good repeatability. HIN had an intermediate repeatability, whereas FRY had the lowest value. The association between HIN and FRY was lower than expected, suggesting that HIN in early stages was not reliable as indirect selection for FRY in later stages. There was a consistent decrease in the average performance of clones grown in PYTs compared with the earlier evaluation of the same genotypes at SRTs. A feasible explanation for this trend is the impact of the environment on the physiological and nutritional status of the planting material and/or epigenetic effects. The usefulness of HIN is questioned. Measuring this variable takes considerable efforts at harvest time. DMC and FRY showed a weak positive association in SRT (r = 0.21) but a clearly negative one at UYT (r = −0.42). The change in the relationship between these variables is the result of selection. In later stages of selection, the plant is forced to maximize productivity on a dry weight basis either by maximizing FRY or DMC, but not both. Alternatively, the plant may achieve high dry root yield by simultaneously attaining “acceptable” (but not maximum) levels of FRY and DMC.

Genetic Variation and Trait Correlations in an East African Cassava Breeding Population for Genomic Selection

“…Root weight as a measurement of yield is known to be polygenic and influenced by environment (Fukuda et al, 2002). According to Barandica et al (2016), measurements of yield and its components are best estimated at later stages of the cultivar selection pipelines, when plot sizes are larger than those in the current study. It is partly for this reason that HI has been proposed as an indirect measurement of yield (Kawano et al, 1987).…”

“…Multi-trait breeding goals are easier to achieve when favorable genetic relationships, arising from linkage or pleiotropy, exist among target traits (Lynch and Walsh, 1998). In cassava, undesirable phenotypic and genotypic correlations have been reported for some important traits (Njoku et al, 2015;Barandica et al, 2016;Esuma et al, 2016). On the other hand, an additive genetic correlation between any two traits implies a relationship between the breeding values of individuals (Bernardo, 2003).…”

“…On the other hand, an additive genetic correlation between any two traits implies a relationship between the breeding values of individuals (Bernardo, 2003). In cassava, undesirable phenotypic and genotypic correlations have been reported for some important traits (Njoku et al, 2015;Barandica et al, 2016;Esuma et al, 2016). For example, an undesirable negative genetic correlation (r g = −0.45) between DMC and total carotenoid content has been observed in African cassava breeding population .…”

Excellence in Cassava Breeding: Perspectives for the Future

“…Epigenetic factors, nutritional and physiological status, pathogens and beneficial endophytes affect the general performance of the same genotype through the sequential stages of evaluation depicted in Figure 1 [19,20]. These features explain the poor correlations observed in the phenotypic evaluation of the same genotypes through these different stages of selection [21] and, to some extent, justify the lengthy evaluation process.…”

Genomic selection for productive traits in biparental cassava breeding populations

“…The use of improved varieties is one option to increase the production of the crop without increasing the planting area. Cassava breeding is based on recurrent phenotypic selection, taking advantage of vegetative propagation [6], and includes several stages of selection [7].…”