Biomedical Engineering Reference
In-Depth Information
over early diploid hybrids, to a majority of diploid hybrids in the current market.
Breeding at the diploid level gives breeding companies greater flexibility in creat-
ing hybrids tailored for particular niches, for example, in markets where rhizomania
is a serious problem and where homozygous genetic resistance is essential in both
seed and pollen parents for effective control. In these cases, marker-assisted
selection for rhizomania resistance is routine (so far, perhaps the only trait in beet
where this is true). Dosage of
Rz1
and
Rz2
genes can be more easily ascertained at
the diploid level, and inbreeding within each of the parental lines can be used to fix
such monogenic traits relatively quickly.
Genetic modification for sugar beet improvement offers the potential for the
introduction of new traits that are not possible to introduce by traditional breeding
approaches. One transgene event (H7-1) for resistance to the glyphosate herbicide
was deregulated in the USA in July 2012. Other traits such as virus (rhizomania)
and fungal (
Cercospora
) resistances are targeted but are not yet being evaluated for
regulatory approval. Transgenic approaches for sugar beet
improvement are
recently reviewed [
60
].
Beta vulgaris
has a haploid genome size of ~750 Mbp and a base chromosome
number of
n
9. Approximately 60 % of the genome consists of moderately to
highly repetitive elements. Many molecular marker maps have been constructed
[
27
]; however, their resolution and their ability to discriminate QTLs (quantitative
trait loci) have been still somewhat limited, and progress has been deferred in
anticipation of higher-density mapping approaches and whole genome sequences.
The beet genome is in the process of being assembled and annotated [
72
,
73
,
114
],
and this will help immeasurably with SNP discovery and perhaps with genotyping-
by-sequencing approaches. Part of the lack of progress in applying markers to beet
breeding is that the majority of breeding materials are self-incompatible and thus
population improvement approaches are most appropriate for new variety devel-
opment. Relatively few inbred lines are available, particularly in the public sector;
thus genetic dissection of traits of interest via conventional Mendelian transmission
genetic approaches requires molecular analyses of sibmated lines or of entire
populations, and these approaches have been prohibitively expensive to date.
Association mapping has been suggested as one means to circumvent this limitation
and is quite promising [
74
], but marker density is still too low for regular discovery
of trait genes via such fine mapping approaches.
Also lacking is knowledge of the genes that control agronomic trait expression in
different environments and during development, although significant progress has
been made via transcript profiling approaches in recent years [
70
,
75
,
76
]. Plant
development occurs under favorable conditions of water, light, and moderate
temperatures, irrespective of most response-to-environment influences, such as
temperature extremes, nutrient and soil (moisture) stress, and pathogen infection.
Response-to-environment genes affect development, but developmental paradigms
should be canonical for crop type and vary mostly in the timing of their expression,
at least as a first approximation. Consequently, transcription profiling methods have
been more successful in defining beet developmental genes and pathways than they
have for
¼
x
¼
response-to-environment genes. Developmental changes are quite
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