Improvement of Yield per se in Sugarcane - Translational Genomics for Crop Breeding: Abiotic Stress, Yield and Quality - page 223

summarizes QTL detection methods and results.
All these QTL studies relied on pseudo 'F2'
populations derived from selfed or bi-parental
crosses of highly heterozygous sugarcane clones.
Depending on the study concerned, popula-
tions were genotyped with different types of
molecular markers (RAPD, RFLP, AFLP, SSR,
TRAP, SRAP, and DArTs). Linkage maps (based
on associations of markers in the coupling
phase), were constructed mainly using single-
dose (SD) markers but occasionally double-
dose (DD) markers (Alwala and Kimbeng 2010;
Wu et al. 1992). Detection of associations of
marker/traits was most often based on marker-
by-marker analysis of variance, or more rarely
on interval mapping. Among genetic maps, the
two most advanced ones for cultivar R570 (Hoa-
rau et al. 2001) and cultivar Q165 (Aitken et al.
2005), which contain around 1,000 markers, still
cover less than 50% of the sugarcane genome
(Piperidis et al. 2008). Two reasons related to the
high polyploidy context may explain this incom-
plete coverage of current interspecific cultivars:
(1) the size of their genome, which is estimated
to be around 17,000 cM (Hoarau et al. 2001), and
(2) the high chromosome redundancy within the
fraction of the genome inherited from S. offici-
narum (about 80%), which implies a lower fre-
quency of single-dose (SD) markers compared to
the frequency of SD markers within the genome
fraction inherited from S. spontaneum (Grivet
et al. 1996; Hoarau et al. 2001; Rossi et al. 2003).

( S. officinarum ) x Mol 5829 ( S. robustum ). They
identified 12 markers linked to five traits asso-
ciated with yield, but the size of the progeny
was small (44) and the P value threshold was
high (0.1). Ming and co-workers (2001) studied
sugar content by measuring sucrose-related traits
(brix and pol) in two interspecific populations −

Green German ( S. officinarum ) x IND81-146
( S. spontaneum ), and PIN 84-1 ( S. spontaneum )
x Muntok Java ( S. officinarum ) of similar size

(264 and 239 individuals respectively). Despite
differences in genotyping efforts between the
two studies, a similarly high percentage of the
total phenotypic variation of sucrose content
was explained in both studies by the markers
detected, which were R 2

−

65% (14 SD markers)
and 68% (22 SD markers), respectively. In these
two populations, Ming and colleagues (2002a,
2002b) also detected a total of 82 QTLs asso-
ciated with traits related to cane yield compo-
nents (stalk weight and height, tillering, and fiber
content). Considering all traits together (sugar
content and cane yield components), individ-
ual R 2 markers frequently ranged from 4% to
about 16% and up to 23% for one particular trait
(Ming et al. 2001; Ming et al. 2002a; Ming et al.
2002b). These relatively high upper values in
individual R 2 ranges can obviously be ascribed
to the interspecific nature of the mapping pop-
ulations that offered large segregations for most
of the traits, possibly due to a few rare alleles of
major effects. Congruently, a second team, also
studying an interspecific cross ( S. officinarum x
S. spontaneum ) (Alwala et al. 2009) reported
markers of high individual effect size (up to
34% for brix content) in a smaller population
(only 100 individuals). In addition, research by
the teams of Alwala and Ming (Ming et al. 2001;
Ming et al. 2002a; Ming et al. 2002b; Alwala
et al. 2009) revealed the existence of unfavor-
able alleles for sucrose content in the favorable
ancestor ( S. officinarum ) and the reverse (favor-
able alleles in S. spontaneum ), even if the direc-
tion of the majority of the QTLs contributing to
trait variation remains congruent with agronomic
predictions. These results suggest the existence

=

InterspecificCrosses

The first category of QTL studies for yield
components was carried out on interspecific
crosses, with the aim of maximizing segrega-
tion of the phenotypic variation in yield. These
studies involved S. officinarum , the domesticated
species, with either the wild species S. robus-
tum in one cross (Sills et al. 1995) or the wild
species S. spontaneum in three crosses (Alwala
et al. 2009; Ming et al. 2002a; Ming et al. 2001;
Ming et al. 2002b). The first research was con-
ducted by Sills and coworkers (1995) on Purple