|Schnell Ii, Raymond|
Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/22/2005
Publication Date: 3/15/2006
Citation: Cervantes-Martinez, C., Phillips-Mora, W., Brown, J.S., Motamayor, J.C., Takrama, J.F., Schnell II, R.J. 2006. Combining ability for disease resistance, yield, and horticultural traits for cacao (Theobroma cacao L.) clones. Journal of the American Society for Horticultural Science. 131(2):231-241. Interpretive Summary: The use of molecular markers to confirm identities of cacao (Theobroma cacao L) clones in germplasm collections and breeding nurseries has recently been found to be essential, as between 12% to 40% of such clones have been found to be mislabeled. Obviously, this has a great effect on the success of any breeding project, and can lead to the loss of considerable amounts of time, effort, and research funding. A partial full-sib design (a set of three recurrent clones each crossed to a set of 10 complementary clones) was used to create a set of crosses, the progeny of which were planted out (32 trees of each cross) in a randomized full-block design with three replications at the “La Lola” research farm of the Tropical Agricultural Research and Higher Education Center (CATIE), Costa Rica. Data was collected for disease resistance to Frosty pod (Moniliophthora roreri) and Black pod diseases, for yield (as measured by pod number), and a number of horticultural traits of interest to breeders. Molecular markers were used to verify the identity of parental trees used for crossing, and several trees were found to be either slight off-types, or to have had completely false identity. Progeny were also checked for crosses made to one clone (UF273), and small numbers of incorrectly pollinated progeny were identified for most crosses excepting one, for which 50% of the progeny were the result of self-pollination. Analysis of the data revealed important estimates of general and specific combining ability for several clones and crosses for several traits. These estimates are important for breeders, in that they provide the breeders with information needed for planning the correct breeding strategy. Heretofore, it was thought that very little heterosis (hybrid vigor), an important factor for successful breeding for most crops, existed in cacao, especially for yield. The results of this analysis showed that heterosis does exist for yield and other traits, as well, and that previous thinking about cacao breeding did not prove true, at least for this set of crosses. As the clones used in this experiment were generally important in world-wide cacao breeding, it is likely that heterosis for yield is more generally important in cacao breeding, as it is in other tree fruit crops with considerable heterozygosity, as is the case for the genetic structure of cacao. In contrast to yield, response to disease resistance of the two diseases measured showed mostly additive gene action. This work provides cacao breeders with the first estimates of general and specific combining ability combined with verification of identity of crosses.
Technical Abstract: Knowledge of genetic differences among commonly cultivated cacao (Theobroma cacao L.) clones, as well as the type of gene action involved, for disease resistance, yield, quality, and horticultural traits are essential for cacao breeders to select parental clones efficiently and effectively. This information is also critical for quantitative geneticists in designing and improving of QTL localization strategies using breeding populations, whether they involve analysis of multiple populations crossed to one common parent or association genetic analysis. The objectives of this research were a) to verify the genetic identity of parental cacao clones used to produce hybrids for field evaluation at CATIE using molecular marker analysis, and b) to estimate general and specific combining ability of the parental clones for resistance to Frosty Pod (Moniliophthora roreri) and Black Pod (Phytophthora palmivora) diseases, pod yield, vigor (as measured by trunk diameter), and measures of maturity. Misidentification of cacao clones occurred at three levels. Six parental clones differed in identity to supposedly identical accessions from other sources. Trees of the parental clone ‘UF 273’ consisted of two clearly different genotypes, resulting in two types of progeny, requiring separate designation for correct statistical analysis. Out-crossed progeny, presumably from foreign pollen, and selfed progeny were also found. Two of the traits measured, percent healthy pods and percent pods with Frosty Pod, showed predominantly additive gene action, while the traits, total number of pods and trunk diameter, demonstrated regulation by both additive and non-additive gene action. Number of months to first flowering and first fruit both showed evidence of predominant regulation by non-additive gene effects. Two parental clones, ‘UF 712’ and ‘UF 273 Type I’, were identified as potential candidates for QTL analysis using breeding populations for the traits, Frosty Pod resistance and total pod production, respectively, given their favorable GCA estimates for these traits.