biomass has to be employed in producing flowers, nectar, and so on. Other disadvantages to
an obligate outcrosser are that if only one genotype is present in an area, the plant may not be
able to reproduce sexually, or reproduction may be rendered uncertain or unlikely by environ-
mental factors. With outcrosssing, each generation produces new variability, and although
most progeny may be fit and well adapted, some progeny may be less fit and constitute
“genetic load” to the population. The third method of reproduction—apomixis—factilitates
the production of a large numberof well-adapted plants of the maternal genotypewith little or
no genetic load. Apomixis offers the possibility of reproduction by seed in plants with “odd”
or unbalanced chromosome numbers, as such plants are unable to produce viable gametes at
meiosis and are likely to be totally or partially seed-sterile. Seed apomixis, for example,
provides all the advantages of the seed habit (dispersal of propagules and a potential
means of survival through unfavorable seasons). Apomicts are often of polyploid and
hybrid origin, and therefore this reproductive mode can potentially serve as a means of pre-
serving high heterozygosity. Apomixis, like selfing, would also appear to be important at the
edge of the range of a species allowing populations to persist in areas in which various factors
may limit or exclude the possibility of sexual reproduction. Given that all three reproductive
modes have advantages and disadvantages depending on environmental circumstances, it is
not too surprising to learn that plants often have highly flexibile mating systems, reproducing
by several means, rather than relying on only a single reproductive mode.
The mating system of a plant species will influence the way in which the genetic diver-
sity present in the species is distributed within and among its populations—specifically, its
population genetic structure. In outcrossing species, higher levels of genetic diversity are
found within than among populations. The opposite is true for predominantly selfing
species where greater among-population (i.e., interpopulation) differentiation is expected.
Knowledge of a plant’s mating system is important in conservation of its genetic diversity
in a seed genebank, or for efficient screening of populations of wild species as source of
traits for crop improvement in plant breeding programs. More populations of a selfing
species would be needed in order to capture the true diversity of a species.
2.4.3 Hybridization and Polyploidy
Although we think of species as discrete and static breeding entities, examples can be found
throughout the angiosperms where different species have the capacity to cross with another.
Plants are champions at interspecific hybridization.Hybridization, or the process of sexual
reproduction between members of different species or biotypes within a species, produces
plants that have genetic material from both parents. In most cases, the initial hybridization
event results in hybrid plants that are haploid for each genome or in other words, have a
single homologous chromosome from each parental chromosome set (Fig. 2.13). As
homologous chromosomes are normally paired during metaphase I, the presence of only
one of each homologous chromosome pair, can disrupt normal meiotic function. In fact,
most of the gametes produced in hybrids are abnormal, leading to sterility to reduced via-
bility of pollen or eggs in the hybrid plant. Although hybrids can be made from the crossing
of many different species, hybridization of normal haploid gametes rarely generates plants
that are fully fertile.
In some cases, sex cells are produced that have more than just one of each homologous
chromosome.Nondisjunction, when homologous chromosomes fail to separate during
meiosis, sometimes generates gametes that have complete sets of chromosomes from the
2.4. PLANT REPRODUCTIVE BIOLOGY 39