Environmental Engineering Reference
In-Depth Information
13.4 cassava BreedInG For ImProved
BIoenerGy ProductIon
The importance of cassava as a global food and fuel source demands the production of improved
germplasm. The International Center for Tropical Agriculture (CIAT), headquartered in Columbia,
established a cassava-breeding program in the early 1970s with the aim of improving yield poten-
tial and tolerance to pests and diseases. Cassava is naturally outbreeding, which when coupled
with possible introgression of wild species germplasm has resulted in highly diverse genotypes. A
lot of genetic diversity has been shown to exist for most traits examined, but extensive inbreeding
depression and the long life cycle make it difficult to develop appropriate stocks for classical genetic
studies (Fregene et al. 2001). As a consequence, studies on the genetics of cassava have been very
limited and genetics of most traits are not well understood. This is especially the case for traits with
relevance to bioenergy production, which are mainly quantitative.
Cassava improvement has continued to tap genetic variation for a long time, mainly through
conventional breeding and to a less extent, using advanced molecular techniques. Clonal selection
has been the predominant method in conventional breeding for cassava improvement at national
centers in Africa and Brazil. The only exception to this is the production of cassava mosaic disease
(CMD)-resistant clones, which has been done by hybridizing
M. glaziovii
with cassava (Storey and
Nichols 1938). The search for varieties that are resistant to various pests and diseases has formed a
major part of cassava research over the last three decades (Ceballos et al. 2004). CMD is the most
important disease of cassava and is the most widespread cassava disease in Africa (Akano et al.
2002). Breeding has tended toward the development of varieties with CMD resistance (Thresh and
Cooter 2005). Several quantitative trait loci (QTL) associated with polygenic and recessive sources
of resistance to CMD have been identified (Lokko et al. 2007).
A few attempts that have been made to understand the genetics of starch, and starch-related traits
have been reported to be polygenically controlled. Kawano et al. (1987) reported polygenic additive
control of root dry matter content in cassava, whereas a nonadditive gene control was demonstrated
for dry matter and starch content (Easwari Amma et al. 1995; Easwari Amma and Sheela 1998)
using classical studies. Several attributes of cassava carbohydrate metabolism suggest that there is
unrealized potential for enhanced starch production (Ihemere et al. 2006). The recent development
of linkage maps and use of molecular markers has made it possible to study and localize quantitative
traits of agronomic importance using QTL mapping. Several QTL for dry matter content have also
been identified in cassava (Kizito et al. 2007). Dry matter content has been found to have a positive
correlation with starch content.
There are currently known expressed sequence tags (ESTs) for specific genes involved in the
starch biosynthesis pathway (Sakurai et al. 2007) that could be used to develop single nucleotide
polymorphisms (SNPs) to improve our understanding of the genetics of starch content and yield in
cassava. The application of the complementary DNA (cDNA)-AFLP (amplified fragment length
polymorphism) technique to generate polymorphic transcript-derived fragments (TDFs) (Suarez
et al. 2000) between the parents of a mapping population has been shown to be a potentially power-
ful way of identifying candidate loci controlling agronomic traits in cassava and could be used to
identify more candidate genes for starch content and yield. There are major efforts toward the devel-
opment of SNPs from ESTs and bacterial artificial chromosome (BAC)-end clones (Lopez et al.
2005; Sakurai et al. 2007) that could be further used for fine-mapping and cloning of the important
traits. Amplification of simple sequence repeats (SSRs) in wild relatives of cassava has been tested
with success (Roa et al. 2000), creating an opportunity for possible exploitation of important starch
traits in the wild species of cassava.
Mutation genetics using irradiation looks promising for creating genetic variation for starch
content in cassava, although it has not yet been exploited. The probable recessive nature of such
mutations will require the need for ways of overcoming inbreeding depression. CIAT has further
implemented different approaches to develop and identify clones with novel starch properties
