Introduction
Cassava (Manihot esculenta Crantz) is the third most
important source of calories in the tropics, after rice
and maize (Huang et al. 2001; Food and Agriculture
Organization of the United Nations (FAO) 2010) and
sixth most important crop in terms of global annual
production (Food and Agriculture Organization of the
United Nations (FAOSTAT) 2010). Widely grown in
tropical Africa, Asia and Latin America, cassava is the
developing world’s fourth most important crop, with
production in 2006 estimated at 226 million tonnes
(FAO 2008, 2005). It is the staple food of nearly a
billion people in 105 countries where the root provides
as much as a third of daily calories (FAO 2008) and the
annual consumption is greatest in Africa. In Zaire for
example, cassava provides for 1,123 calories per day
per person while in Ghana, a mean per capita
production of 465 kg/annum provides about 20 % of
calories in the diet, far ahead of any other single crop
or animal source (FAOSTAT 2005). It is also a major
staple for about half of the Nigerian population and
nearly 50 million tonnes of fresh cassava is produced
annually in the country (FAOSTAT 2008). Cassava is
important not just as a subsistence or food security
crop, but also as a main source of cash income for
producing households (Bainbridge et al. 1997). Its
production and processing provide employment and
income for the rural poor, especially women and
children (Sarma and Kunchai 1989).
According to Nweke et al. (1994), cassava’s long
growth cycle makes it relatively difficult for the crop
to be readily available on time to farmers and
consumers and rainfall pattern in some parts of the
tropics where rain lasts for about 6 months or less
provides distinction between rainy and dry seasons
thereby making cassava cultivation in such areas
difficult. They also stated that late bulking (LB) is a
major factor leading to rejection and abandoning of
improved cassava genotypes in sub-Saharan Africa
due to demographic and market pressures while early
bulking (EB) is one of the important traits of interest to
farmers in cassava growing regions. It has been found
out that early yield in cassava is highly influenced by
each of harvest index, foliage, root diameter and root
number (Okogbenin et al. 2006). EB varieties shorten
the growth period from planting to harvesting and
better fit into environments with short rainy season,
reduce exposure to biotic and abiotic stresses thereby
increasing productivity (Nweke et al. 1994). Therefore,
identification of molecular markers associated
with EB in cassava may enhance breeding for early
maturing (EM) cassava varieties which will be more
acceptable to the farmers.
Major genes responsible for economically important
characters are frequent in the plant kingdom
(Wikipedia.org 2013). However, these genes are usually
present in different individuals of a particular
species. Recently developed genomics techniques take
advantage of parallel genotyping of single feature
polymorphisms (SFPs) and other small differences
between two strain backgrounds (Deutschbauer and
Davis 2005). One of the tools used to identify
molecular markers associated with a trait in an
organism is bulked segregant analysis (BSA) which
measures the variation present in pools of segregants
that have been sorted according to phenotype and uses
the correlation between these measurements and the
pool phenotype to assign a likely map location (Brauer
et al. 2006). BSA relies on informative individuals
being grouped so that a particular genomic region is
studied against a randomized genetic background of
unlinked loci (Michelmore et al. 1991). This is an
improvement over methods that require individual
genotyping, as it simultaneously measures the average
genotype of progeny with a given phenotype (Brauer
et al. 2006). Also, the simplicity and low cost of BSA
have led to its use for complex traits including those
whose genetic control is unknown and is being used
increasingly to screen for quantitative trait loci (QTL)
(Mackay and Caligari 2000). Two contrasting bulks
for a trait in BSA will differ only at the locus harbouring
that trait and the minimum size of the samples
comprising the bulks is generally determined by the
frequency with which these unlinked loci might be
detected as polymorphic between the two bulks
(Mackay and Caligari 2000). While biotechnology
Introduction
Cassava (Manihot esculenta Crantz) is the third most
important source of calories in the tropics, after rice
and maize (Huang et al. 2001; Food and Agriculture
Organization of the United Nations (FAO) 2010) and
sixth most important crop in terms of global annual
production (Food and Agriculture Organization of the
United Nations (FAOSTAT) 2010). Widely grown in
tropical Africa, Asia and Latin America, cassava is the
developing world’s fourth most important crop, with
production in 2006 estimated at 226 million tonnes
(FAO 2008, 2005). It is the staple food of nearly a
billion people in 105 countries where the root provides
as much as a third of daily calories (FAO 2008) and the
annual consumption is greatest in Africa. In Zaire for
example, cassava provides for 1,123 calories per day
per person while in Ghana, a mean per capita
production of 465 kg/annum provides about 20 % of
calories in the diet, far ahead of any other single crop
or animal source (FAOSTAT 2005). It is also a major
staple for about half of the Nigerian population and
nearly 50 million tonnes of fresh cassava is produced
annually in the country (FAOSTAT 2008). Cassava is
important not just as a subsistence or food security
crop, but also as a main source of cash income for
producing households (Bainbridge et al. 1997). Its
production and processing provide employment and
income for the rural poor, especially women and
children (Sarma and Kunchai 1989).
According to Nweke et al. (1994), cassava’s long
growth cycle makes it relatively difficult for the crop
to be readily available on time to farmers and
consumers and rainfall pattern in some parts of the
tropics where rain lasts for about 6 months or less
provides distinction between rainy and dry seasons
thereby making cassava cultivation in such areas
difficult. They also stated that late bulking (LB) is a
major factor leading to rejection and abandoning of
improved cassava genotypes in sub-Saharan Africa
due to demographic and market pressures while early
bulking (EB) is one of the important traits of interest to
farmers in cassava growing regions. It has been found
out that early yield in cassava is highly influenced by
each of harvest index, foliage, root diameter and root
number (Okogbenin et al. 2006). EB varieties shorten
the growth period from planting to harvesting and
better fit into environments with short rainy season,
reduce exposure to biotic and abiotic stresses thereby
increasing productivity (Nweke et al. 1994). Therefore,
identification of molecular markers associated
with EB in cassava may enhance breeding for early
maturing (EM) cassava varieties which will be more
acceptable to the farmers.
Major genes responsible for economically important
characters are frequent in the plant kingdom
(Wikipedia.org 2013). However, these genes are usually
present in different individuals of a particular
species. Recently developed genomics techniques take
advantage of parallel genotyping of single feature
polymorphisms (SFPs) and other small differences
between two strain backgrounds (Deutschbauer and
Davis 2005). One of the tools used to identify
molecular markers associated with a trait in an
organism is bulked segregant analysis (BSA) which
measures the variation present in pools of segregants
that have been sorted according to phenotype and uses
the correlation between these measurements and the
pool phenotype to assign a likely map location (Brauer
et al. 2006). BSA relies on informative individuals
being grouped so that a particular genomic region is
studied against a randomized genetic background of
unlinked loci (Michelmore et al. 1991). This is an
improvement over methods that require individual
genotyping, as it simultaneously measures the average
genotype of progeny with a given phenotype (Brauer
et al. 2006). Also, the simplicity and low cost of BSA
have led to its use for complex traits including those
whose genetic control is unknown and is being used
increasingly to screen for quantitative trait loci (QTL)
(Mackay and Caligari 2000). Two contrasting bulks
for a trait in BSA will differ only at the locus harbouring
that trait and the minimum size of the samples
comprising the bulks is generally determined by the
frequency with which these unlinked loci might be
detected as polymorphic between the two bulks
(Mackay and Caligari 2000). While biotechnology
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