Agriculture Reference
In-Depth Information
Techniques used for producing haploids
in vitro
Although haploidy is a very attractive technique to many
plant breeders the natural occurrence of haploid plants is
rare. However, the use of plant tissue culture has allowed
the production of plants from gametic cells cultured
in vitro
.
Although haploid plants can be regenerated from
both male and female sex cells, it is generally the male
cells (microspores or pollen) that have proven most suc-
cessful in the regeneration of large numbers of haploid
and doubled haploid lines. This is partly because of the
ease with which pollen, as opposed to eggs can be col-
lected, and partly because it is simply that, in general,
many more pollen grains than eggs are produced.
There are, of course exceptions and some examples
include:
•
Raphanobrassica
, which resulted from the inter-
generic cross between kale (
Brassica oleracea
) and
radish (
Raphanus sativus
)
When each of these new species was created there
was great hope that they would have almost immediate
high potential commercial value. However, in neither
instance has this full commercialization occurred - at
least not yet.
TISSUE CULTURE
A variety of techniques have been developed under the
title of tissue culture. It is not the intention to cover
the details of these techniques but to briefly consider a
couple of them, enough to be able to give an idea of
their application.
The relative ease by which haploid barley plants
can be produced from female sex cells. Interspecific
crosses between cultivated barley (
Hordeum vulgare
)
and the wild species
H. bulbosum
followed by
in vitro
culture of rescued immature embryos results in hap-
loid plants as a result of exclusion of the
H. bulbosum
chromosomes during embryo development.
•
Haploidy
Establishing true breeding, homozygous lines (as noted
earlier), is an essential part of developing new cultivars
in many crop species. These homozygous lines are used
either as cultivars in their own right or as parents in
hybrid variety development. Traditionally, plant breed-
ers have used the process of selfing or mating between
close relatives to achieve homozygosity, a process that
is time consuming. Therefore the opportunity to pro-
duce plants from gametic, haploid cells has been the
goal of many plant breeders as this technique would
produce 'instant' inbred lines once the chromosomes of
the haploids are doubled.
The genetic phenomenon critical to obtaining
homozygous lines is the formation of haploid gametes
by meiosis. During this type of cell division, the chro-
mosome number is halved and each chromosome is
represented only once in each cell (assuming the species
is basically a diploid one). If such gametic, haploid cells
can be induced to develop into plantlets (i.e. we encour-
age the development of the sporophyte -
note
:lower
plants often have this as a specific phase of the life-cycle)
a haploid plant can develop which can then be treated
(usually with a chemical called colchicine) to encourage
its chromosomes to double, to produce a completely
homozygous line (a doubled haploid).
•
Dihaploids from tetraploid potatoes have been
produced in large quantity, using interspecific
hybridization between cultivated potato (
Solanum
tuberosum
) and a diploid relative (
S. phureja
). The
cross of the tetraploid female
S. tuberosum
with
the diploid male
S. phureja
would be expected to
produce only triploid offspring - but it does not.
Instead, the numbers of seeds obtained are rela-
tively few and are predominantly tetraploid (as a
result of the production of unreduced (2
n
) pollen
from
S. phureja
). Among the rest are some of the
expected sterile triploids but also some maternal
dihaploids arising from the egg. Lines of
S. phureja
have been selected which produce a high frequency
of dihaploid seed, greater than 70%. In addition
such pollen parents have been selected to include a
homozygous dominant embryo spot marker, which
makes visual identification of the non-dihaploid
seed easy.
There are other haploid induction mechanisms but
the most widely applicable are via anther or microspore
(immature
pollen
grains)
in
vitro
.
The
anthers,
of course,
are flower organs in which microspores
