Principles of Plant Genetics and Breeding

64 CHAPTER 4
price of hybrid seed. Apomixis, as previously indicated,
accelerates plant breeding. This could translate into
less expensive commercial seed for all producers.
Realistically, such benefits will materialize only if commercial
breeders can make an acceptable profit from
using the technology.
Impact on the environment Some speculate that
apomixis has the potential to reduce biodiversity
because it produces clonal cultivars and hence uniform
populations that are susceptible to disease epidemics.
However, others caution that the suspected reduction in
biodiversity would be minimal since apomixis occurs
naturally in polyploids, which occur less frequently than
diploids.
Mechanisms of apomixis
Apomixis arises by a number of mechanisms of which
four major ones that differ according to origin (the cell
that undergoes mitosis to produce the embryo) are discussed
next. Seed formation without sexual union is
called agamospermy, a mechanism that can be summarized
into two categories: gametophytic apomixis and
adventitious apomixis. There are two types of gametophytic
apomixes: apospory and diplospory.
1 Apospory. This is the most common mechanism of
apomixis in higher plants. It is a type of agamospermy
that involves the nucellar. The somatic cells of the
ovule divide mitotically to form unreduced (2n)
embryonic sacs. The megaspore or young embryo
sac aborts, as occurs in species such as Kentucky
bluegrass.
2 Diplospory. An unreduced megaspore mother cell
produces embryo sacs following mitosis instead of
meiosis. This cytological event occurs in species such
as Tripsacum.
3 Adventitious embryo. Unlike apospory and diplospory
in which an embryo sac is formed, no embryo
sac is formed in adventitious embryony. Instead, the
source of the embryo could be somatic cells of the
ovule, integuments, or ovary wall. This mechanism
occurs commonly in citrus but rarely in other higher
plants.
4 Parthenogenesis. This mechanism is essentially
equivalent to haploidy. The reduced (n) egg
nucleus in a sexual embryo sac develops into a
haploid embryo without fertilization by the sperm
nucleus.
Other less common mechanisms of apomixis are
androgenesis (development of a seed embryo from
the sperm nucleus upon entering the embryo sac) and
semigamy (sperm nucleus and egg nucleus develop
independently without uniting, leading to a haploid
embryo). The resulting haploid plants contain sectors of
material from both maternal and paternal origin.
Industry highlights
Maize × Tripsacum hybridization and the transfer of apomixis:
historical review
Bryan Kindiger
USDA-ARS Grazinglands Research Laboratory, El Reno, OK 73036, USA
Research in maize–Tripsacum hybridization is extensive and encompasses a period of more than 60 years of collective research.
The publication The origin of Indian corn and its relatives describes some of the initial research in this area (Mangelsdorf & Reeves
1939) and is recommended reading for anyone interested in this area of research. Since this historic publication, an abundance of
literature has been developed with regard to the various facets of this type of hybridization ranging from agronomy, plant disease,
cytogenetics, and breeding, to genetic analysis. As a consequence, no single article can cover all the research relevant to this
topic. This report will only briefly highlight a specific series of experiments that address an attempt to investigate the transfer of
apomixis from Tripsacum dactyloides to Zea mays.
One of the most interesting instances of intergeneric hybridization involves hybridizing maize (Z. mays L.) (2n = 2x = 20) with
its distant relative eastern gamagrass (T. dactyloides) (2n = 4x = 72). Regardless of their complete difference in chromosome
number, plant phenotype, and environmental niche, hybrids are relatively easy to generate. The F 1 hybrids are completely
pollen-sterile and microsporogenesis is associated with a varying array of meiotic anomalies (Kindiger 1993). The hybrids vary in
seed fertility ranging from completely sterile to highly seed-fertile (Harlan & de Wet 1977). To date, all seed-fertile hybrids generated
from tetraploid T. dactyloides exhibit some level of apomictic expression; however, backcrossing with maize commonly
results in the loss of apomixis.