Cassava Breeding I: The Value of Breeding Value

Abstract

Breeding cassava relies on several selection stages (single row trial-SRT; preliminary; advanced; and uniform yield trials—UYT). This study uses data from 14 years of evaluations. From more than 20,000 genotypes initially evaluated only 114 reached the last stage. The objective was to assess how the data at SRT could be used to predict the probabilities of genotypes reaching the UYT. Phenotypic data from each genotype at SRT was integrated into the selection index (SIN) used by the cassava breeding program. Average SIN from all the progenies derived from each progenitor was then obtained. Average SIN is an approximation of the breeding value of each progenitor. Data clearly suggested that some genotypes were better progenitors than others (e.g., high number of their progenies reaching the UYT), suggesting important variation in breeding values of progenitors. However, regression of average SIN of each parental genotype on the number of their respective progenies reaching UYT resulted in a negligible coefficient of determination (r2 = 0.05). Breeding value (e.g., average SIN) at SRT was not efficient predicting which genotypes were more likely to reach the UYT stage. Number of families and progenies derived from a given progenitor were more efficient predicting the probabilities of the progeny from a given parent reaching the UYT stage. Large within-family genetic variation tends to mask the true breeding value of each progenitor. The use of partially inbred progenitors (e.g., S1 or S2 genotypes) would reduce the within-family genetic variation thus making the assessment of breeding value more accurate. Moreover, partial inbreeding of progenitors can improve the breeding value of the original (S0) parental material and sharply accelerate genetic gains. For instance, homozygous S1 genotypes for the dominant resistance to cassava mosaic disease (CMD) could be generated and selected. All gametes from these selected S1 genotypes would carry the desirable allele and 100% of their progenies would be resistant. Only half the gametes produced by the heterozygous S0 progenitor would carry the allele of interest. For other characteristics, progenies from the S1 genotypes should be, at worst, similar to those generated by the S0 progenitors.

Induction of flowering in cassava through grafting

“…The impact of conventional breeding would be increased particularly if inbreeding could be incorporated into the process (Ceballos et al, 2015(Ceballos et al, , 2016. The implementation of genomic selection would benefit by inducing early flowering, a fact that was recognized by the Next Generation Cassava Breeding project www.nextgencassava.org).…”

“…Moreover, the need for a protocol to accelerate flowering in cassava has become more urgent in recent years. The advantages to introduce inbreeding in cassava genetic enhancement have been demonstrated (Ceballos et al, 2015(Ceballos et al, , 2016. Accelerated flowering would facilitate the development of inbred progenitors through successive self-pollinations.…”

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

“…The same data presented here was used for a “sister” article attempting to identify factors in SRT that could predict probabilities of genotypes reaching the UYT stage (Ceballos et al, 2016). All available data has been used here for each analysis to maximize the degrees of freedom and avoid any undesirable bias in the analysis.…”

“…Large experimental errors, genotype by environment interaction, the influence of quality of planting material (and other epigenetic effects) and the changing nature of the association between key traits, such as FRY and DMC are some of the problems faced by cassava breeders, particularly when selecting in the large SRT. The use of breeding value of progenitors based on average SIN (proposed by (Ceballos et al, 2004)), seems to offer limited benefits (Ceballos et al, 2016). Results presented here provide feasible explanations for the conclusions reached in that “ sister ” article.…”

“…A large amount of information has been generated and can be used to relate the information generated by the germplasm reaching the multi-location stage of evaluation with all the previous stages of selection. An article has already been published using the same data (Ceballos et al, 2016). This knowledge is crucial for determining which variables are more efficiently selected for at different stages of selection.…”