Principles of Plant Genetics and Breeding

Apomixis
Seed production in higher plants that are sexually propagated
species normally occurs after a sexual union in
which male and female gametes fuse to form a zygote,
which then develops into an embryo. However, some
species have the natural ability to develop seed without
fertilization, a phenomenon called apomixis. The consequence
of this event is that apomictically produced
seeds are clones of the mother plant. That is, apomixis is
the asexual production of seed. Unlike sexual reproduction,
there is no opportunity in apomixis for new recombination
to occur to produce diversity in the offspring.
Occurrence in nature
Apomixis is widespread in nature, having been found in
unrelated plant families. However, it is infrequent in
occurrence. About 10% of the estimated 400 plant families
and a mere 1% of the estimated 40,000 species they
comprise exhibit apomixis. The plant families with the
highest frequency of apomixis are Gramineae (Poaceae),
Compositae, Rosaceae, and Asteraceae. Many species
of citrus, berries, mango, perennial forage grasses, and
guayule reproduce apomictically.
Some species can produce both sexual and apomictic
seeds and are called facultative apomicts (e.g., bluegrass,
Poa pratensis). Species such as bahiagrass
(Paspalum notatum) reproduce exclusively or nearly so
by apomixis and are called obligate apomicts. There are
several indicators of apomixis. When the progeny from
a cross in a cross-pollinated (heterozygous) species fails
to segregate, appearing uniform and identical to the
mother plant, this could indicate obligate apomixis.
Similarly, when plants expected to exhibit high sterility
(e.g., aneuploids, triploids) instead show significantly
high fertility, apomixis could be the cause. Obligate
apomicts may display multiple floral features (e.g., multiple
stigmas and ovules per floret, double or fused
ovaries), or multiple seedlings per seed. Facultative
apomixis may be suspected if the progeny of a cross
shows an unusually high number of identical homozygous
individuals that resemble the mother plant in
addition to the presence of individuals that are clearly
different (hybrid products). Using such morphological
indicators to discover apomicts requires keen observation
and familiarity with the normal breeding behavior
of the species.
The indicators suggested are by no means conclusive
evidence of apomixis. To confirm the occurrence of
apomixis and discovery of its mechanisms requires
PLANT REPRODUCTIVE SYSTEMS 63
additional progeny tests as well as cytological tests of
megasporogenesis and embryo sac development.
Benefits of apomixis
The benefits of apomixis may be examined from the perspectives
of the plant breeder and the crop producer.
Benefits to the plant breeder Apomixis is a natural
process of cloning plants through seed. As a breeding
tool, it allows plant breeders to develop hybrids that can
retain their original genetic properties indefinitely with
repeated use, without a need to reconstitute them. In
other words, hybrid seed can be produced from hybrid
seed. The plant breeder does not need to make crosses
each year to produce the hybrid. This advantage accelerates
breeding programs and reduces development costs
of hybrid cultivars. Apomixis is greatly beneficial when
uniformity of product is desired. Breeders can use this
tool to quickly fix superior gene combinations. That is,
vigor can be duplicated, generation after generation
without decline. Furthermore, commercial hybrid production
can be implemented for species without fertility
control mechanisms (e.g., male-sterility system), neither
is there a need for isolation in F 1 hybrid seed production.
There is no need to maintain and increase parental
genotypes. Cultivar evaluation can proceed immediately
following a cross.
Apart from these obvious benefits, it is anticipated
that plant breeders will divert the resources saved
(time, money) into other creative breeding ventures.
For example, cultivars could be developed for smaller
and more specific production environments. Also, more
parental stock could be developed to reduce the risk
of genetic vulnerability through the use of a few elite
genetic stocks as parents in hybrid development.
There are some plant breeding concerns associated
with apomixis. Species that exhibit facultative apomixis
are more challenging to breed because they produce
both sexual and apomitic plants in the progeny.
Obligate apomicts are easier to breed by conventional
methods provided compatible (asexually reproducing)
counterparts can be found.
Benefits to the producer The most obvious benefit of
apomitic cultivars to crop producers is the ability to
save seed from their field harvest of hybrid cultivars for
planting the next season. Because apomixis fixes hybrid
vigor, the farmer does not need to purchase fresh hybrid
seed each season. This especially benefits the producer
in poor economies, who often cannot afford the high