Agriculture Reference
In-Depth Information
underlying a linkage map derived from inheritance statistics. Separate 'linkage
groups' of characteristics that are inherited together, each corresponding to a
different chromosome, should result from a linkage map based on many
characteristics. The independent inheritance of characteristics determined by
genes in different linkage groups reflects the random assortment of the two
segregants from each chromosome going to each daughter cell during meiosis.
Considerable research effort is now being devoted to the development of genetic
linkage maps for the edible alliums, particularly onion, as reviewed in McCallum
(2007) and Havey
et al.
(In press).
The first genetic maps were based on the inheritance of easily observable
characteristics but, as we have seen, there are relatively few of these known for
alliums. Moreover, some of the observable characteristics, like disease resistance,
need considerable effort to assess. Nowadays, genetic maps are based primarily
on molecular 'markers', which can be found in large numbers and which can be
rapidly detected using techniques of molecular biology. One of the useful
outcomes of such work is that the inheritance in plant breeding lines of important
properties that need much effort to assess - for example, disease resistance or the
Ms
genes for fertility restoration (see below; Gokce
et al
., 2002) - can be followed
easily by using molecular markers closely linked to a desired trait to rapidly and
cheaply detect its presence or absence in individual plants in a breeding
programme. Often, such tests can be done at the seedling stage, so that only
individuals retaining alleles for a desired trait need be grown on. This is termed
'marker-assisted breeding'. Molecular markers used in the genetic mapping of
alliums were described by Klaas and Friesen (2002). Briefly, molecular markers
group into isoenzymes and DNA-based techniques. Isozymes are allelic variants
of particular enzymes that can be distinguished and followed in inheritance
studies (Shigyo
et al
., 1996).
DNA-based techniques break down into a number of subcategories. First
there are techniques based on differences in the sizes of DNA fragments. These
are generated by applying a succession of cleaving enzymes to the DNA.
Genotypes may differ in the resulting size of DNA fragments from a particular
point along the chromosomes, and the inheritance of these differences in size
fragments can be followed. Secondly there are techniques based on DNA
sequence differences that vary between genotypes. For example, there may be
sequence repeats, nucleotide omissions or nucleotide substitutions. These
molecular variants that show up as detectable DNA differences at the same
location along the homologous chromosomes are called polymorphisms. The
inheritance of the different variants within a polymorphism can be followed,
just as can the inheritance of different alleles for contrasting forms of a visible
attribute like long or short flower stalks.
The first linkage map of onion was based on DNA polymorphisms and has
114 loci located over 12 linkage groups (King
et al
., 1998). It was derived by
following the inheritance of the DNA markers in 58 third-generation families
of controlled crosses and selfs derived from an original cross between an inbred
