Assessment of genetic diversity, combining ability, stability and farmer preference of cassava germplasm in Ghana

Abstract

English: The aim of this study was to assess genetic diversity, combining ability, heterosis, heritability of traits, stability and farmer preference of cassava germplasm in Ghana in order to structure an effective breeding programme. The assessment of genetic diversity in a collection of local cassava genotypes using 33 SSR markers revealed a high overall heterozygosity. The SSR markers identified 63 duplicates in the collection. The heterotic pools were distinct for the savannah and the forest ecological zones and this was the basis for the selection of progenitors for the rest of the study. Seven and five cassava progenitors were crossed in a diallel design without reciprocals in the forest and savannah ecologies respectively. The progenitors and progenies were evaluated for CMD, CBB, yield and its components in two seasons in the forest and one season in the savannah zone. The GCA:SCA ratio indicated that the GCA was larger than SCA for average root number, indicating additive gene effects and a possibility for improvement by selection. SCA was larger than GCA for harvest index and fresh root yield, indicating environmental effects on the trait and confirms the non-additive effects mainly determining expression of root yield. The parental genotypes were selected from a particular region where CMD is the main economic constraint. Debor had the best GCA effect for cassava mosaic severity reaction; while Tuaka had the best GCA effect for fresh root weight. Debor x Kwasea was the best combination for CMD resistance. Debor and Kwasea were the best parents for resistance breeding to CMD. The predictability ratio varied between 0.58 and 0.84. CMD and root number had a ratio close to one for the savannah and forest zones, showing that these traits were highly heritable and influenced by additive gene action. Marker assisted selection was used to search for new sources of CMD resistance in the progenies and progenitors. Crosses among CIAT elite genotypes and landraces revealed progenitor Dabodabo as a putative source of new genes to CMD resistance.

Afrikaans: Die doel van hierdie studie was om genetiese divers iteit, kombineervermoë, heterose, oorerflikheid van eienskappe, stabiliteit en die bo ere se voorkeur vir cassava kiemplasma in Ghana te evalueer sodat ‘n effektiewe teelprogram g estruktureer kan word. Die evaluasie vir genetiese diversiteit met 33 SSR merkers in ‘n vers ameling van plaaslike cassava genotipes het baie heterosigose getoon. Die SSR merkers het 63 du plikate in die versameling uitgewys. Die heterotiese groepe was duidelik verskillend vir die savannah en die woud ekologiese sones en dit was die basis van seleksie van ouers in hierdie stu die. Sewe en vyf cassava ouers onderskeidelik is gekruis in ‘n dialleel ontwerp sonder resiproke, in die woud en savannah ekologiese sones. Die ouers en die nageslag is geëvalueer vir CMD, CBB, o pbrengs en die opbrengs komponente in twee seisoene in die woud en een seisoen in die sav annah sone. Die GCA:SCA verhouding het gewys dat die GCA groter was as SCA vir die aantal wortels, wat additiewe geenaksie wys, dus is verbetering moontlik met seleksie. SCA waardes w as hoër as GCA vir oesindeks en vir vars wortel opbrengs wat wys dat daar groot omgewingseff ekte op opbrengs is en dat nie-additiewe geeneffekte ‘n groot rol speel. Die ouers is gesele kteer uit ‘n spesifieke gebied waar CMD die grootste ekonomiese beperking is. Debor het die bes te GCA effekte vir CMD reaksie gehad; terwyl Tuaka die beste GCA effekte vir vars wortel opbrengs gehad het. Debor x Kwasea was die beste kombinasie vir CMD weerstand. Debor en Kw asea was die beste ouers vir CMD weerstandsteling. Die voorspelbaarheidsverhouding h et gevarieer tussen 0.58 en 0.84. CMD en aantal wortels het ‘n verhouding van na aan een geh ad vir die woud en savannah sones, wat getoon het dat hierdie eienskappe hoogs oorerflik i s, en deur additiewe geenaksie bepaal word. Merker ondersteunde teling en soeke na nuwe bronne van CMD weerstand in die ouers en nageslag is gedoen in kruisings tussen CIAT elite g enotipes en landrasse. Die ouer Dabodabo is as moontlike bron van nuwe CMD weerstandsgene uitge wys.