Agriculture Reference
In-Depth Information
line from the low-pungency, low-soluble solid cv. 'Ailsa Craig' and an inbred
from the higher-pungency and soluble solid cv. 'Brigham Yellow Globe'. Alleles
for male fertility restoration, a complementary gene for red bulb colour (see
previous section) and the enzyme coding for alliinase (see Fig. 8.2) were located
on the linkage map and associated with neighbouring DNA markers. A sub-
sequent analysis of this cross followed the inheritance of DNA markers in
parallel with differences in sugar and fructan (see Chapter 8) content deriving
from the parent lines. Results showed chromosome regions on linkage groups
A, D and E to be associated with the control of fructan content (Havey
et al
.,
2004).
Another genetic map based on a cross between onion and the wild species
A. roylei
discriminated 262 DNA markers on the onion genome into eight
linkage groups (van Heusden
et al
., 2000b). A gene for alliinase and the location
of a gene for resistance to downy mildew disease derived from
A. roylei
were
located by this map. Linkage groups in this map were later assigned to the
individual physical chromosomes of
A. cepa
(see Fig. 3.4a; van Heusden
et al
.,
2000a). This was done using molecular markers for
A. cepa
that could be located
in the
A. cepa
A. roylei
linkage map and also in chromosomes derived from
A.
cepa
in a set of eight lines of bunching onion,
A. fistulosum
, each of which
contained an extra different chromosome from shallot (
A. cepa
) (Shigyo
et al
.,
1996). These lines, termed 'monosomic addition lines', are the key to
translating the linkage map into a 'physical map' located on the eight
chromosomes of
A. cepa
(see Fig. 3.1). Analysis of the biochemical effects of
different monosomic additions has shown that a group for flavonoid and
anthocyanin biosynthesis is on chromosome 5A (Shigyo
et al
., 1997), and
genes for the production of sugars in leaves on chromosomes 2A and 8A (Tran
Thi Minh Hang
et al
., 2004).
A further refinement has been to compare 'genetic distances' between DNA
markers - as shown by linkage maps - with the physical distance between
markers along the chromosomes by direct visualization and position measure-
ment of meiotic crossing over, using a refined staining technique, in parallel with
the recombination mapping of markers (see Fig. 3.4; Khrustaleva
et al
., 2005).
Recombination frequency varied in different regions along the chromosome,
indicating that the 'genetic distances' between markers correspond to variable
physical distances along the chromosome (see Fig. 3.4c). The pattern of variation
of recombination frequency was different to that found for cereal crops.
DNA sequence repeat markers that can be amplified by the polymerase
chain reaction (PCR) have been developed for onion (McCallum
et al
., 2005b).
Polymorphisms for these markers have been detected both between and within
onion breeding lines. Onion linkage maps derived from different original parent
crosses have been aligned using markers of this type to locate common points on
both maps (McCallum
et al
., 2005b). New families of inbred lines from crosses
between parents with contrasting properties have been developed, so that these
markers can be exploited to map the genes underlying these contrasts. Families
