The Use of Association Genetics Approaches in Plant Breeding - Plant Breeding Reviews - page 44

cocoyam or taro, yam, and sweet potato. Root and tuber crops are
clonally propagated, dif

cult to inbreed, and mostly characterized by
high heterozygosity and polyploidy, which make AM studies more
dif cult in these crops as compared with inbred diploid species with

lower heterozygosity.
Among the root and tuber crops, AM studies have only really started
with potatoes, so we will concentrate on this crop. Heterozygosity in
potato is explained by the selective advantage of maximizing high-level
intra- and interlocus interactions for important traits such as tuber yield
(Mendoza and Haynes 1976). Clonal propagation perpetuates heterozy-
gosity and keeps allelic diversity in the populations high. Allelic diver-
sity might be also a key component for potato survival. The cultivated
potato Solanum tuberosum includes diploid and tetraploid species and
most genetic studies so far have used F 1 populations.
Genetic study of heterozygous individuals increases the complexity of
trait analysis, since several allelic combinations (di-, tri-, and tetra-) are
found in segregating populations for each individual, especially in
tetraploids. The study of additive genetic variance is dif

cult in potato

since near isogenic lines are dif cult to develop. Moreover, the number

of alleles increases the dif culty of genotypic and phenotypic charac-

terization (Van Eck 2007). On the other hand, this diversity is important
for AM studies since these studies are based on the existence of diversity
within a species. Heterozygosity can be therefore an advantage and not a
constraint to understanding complex traits in potato and other root and
tuber crops by using AM approaches. Working with heterozygous crops
is convenient for AM, because the high heterozygosity avoids the
problem of subpopulation structure although with the dif

culty and

challenges in finding associations with single alleles that may be domi-

nant or codominant (D ' hoop et al. 2008). Heterozygous populations do

not have high- cients because alleles are being effectively

interchanged avoiding strong subdivisions in populations.
Association genetic research for Solanum potatoes can be simpli

xation coef

ed
by working at the diploid level. This is one advantage of working with
S. tuberosum group Phureja , an Andean potato with small tubers (but
day length sensitivity) that is diploid compared with most (but not all)
S. tuberosum group Tuberosum potatoes that are tetraploid. An alterna-
tive is to select germplasm panels of only diploid S. tuberosum , which
are popular in the Andean region of South America as very important
genetic stocks for genomic research (Potato Sequencing Consortium).
Both types of potatoes are commonly used in breeding programs, but
diploid potatoes facilitate scienti

c analysis as genotyping and pheno-
typing are simpler. Also the work with diploids makes data analysis