Principles of Plant Genetics and Breeding

of all gene loci and traits of the sporophyte.
Consequently, should there be cross-pollination the
resulting heterozygosity is rapidly eliminated. To be
classified as self-pollinated, cross-pollination should not
exceed 4%. The genotypes of gametes of a single plant
are all the same. Further, the progeny of a single plant is
homogeneous. A population of self-pollinated species,
in effect, comprises a mixture of homozygous lines.
Self-pollination restricts the creation of new gene combinations
(no introgression of new genes through
hybridization). New genes may arise through mutation,
but such a change is restricted to individual lines or
the progenies of the mutated plant. The proportions of
different genotypes, not the presence of newly introduced
types, define the variability in a self-pollinated
species. Another genetic consequence of self-pollination
is that mutations (which are usually recessive) are readily
exposed through homozygosity, for the breeder or
nature to apply the appropriate selection pressure on.
Repeated selfing has no genetic consequence in selfpollinated
species (no inbreeding depression or loss of
vigor following selfing). Similarly, self-incompatibility
does not occur. Because a self-pollinated cultivar is generally
one single genotype reproducing itself, breeding
self-pollinated species usually entails identifying one
superior genotype (or a few) and multiplying it. Specific
breeding methods commonly used for self-pollinated
species are pure-line selection, pedigree breeding, bulk
populations, and backcross breeding (see Chapter 16).
Cross-pollinating species
Mechanisms that favor cross-pollination
Several mechanisms occur in nature by which crosspollination
is ensured, the most effective being dioecy.
As previously noted, dioecious species are those in
which a plant is either female or male but not a
hermaphrodite (e.g., hemp, date, palm). When such
species are cultivated from grain or fruit, it is critical
that the producer provides pollinator rows. A less stringent
mechanism is monoecy (i.e., monoecious plants).
Monoecious species can receive pollen from their own
male flowers. Dichogamy occurs in hermaphroditic
flowers, whereby cross-pollination may be enforced when
the stamens mature before the pistil is mature and receptive
(a condition called protandry) or the reverse (called
protogyny). Sometimes, the pollen from a flower is not
tolerated by its own stigma, a condition known as selfincompatibility.
Male sterility, the condition whereby
PLANT REPRODUCTIVE SYSTEMS 61
Table 4.3 Examples of predominantly cross-pollinated
species.
Common name Scientific name
Alfalfa Medicago sativa
Annual ryegrass Lolium multiflorum
Banana Musa spp.
Birdsfoot trefoil Lotus corniculatus
Cabbage Brassica oleracea
Carrot Dacus carota
Cassava Manihot esculentum
Cucumber Cucumis sativus
Fescue Festuca spp.
Kentucky bluegrass Poa pratensis
Maize Zea mays
Muskmelon Cucumis melo
Onion Allium spp.
Pepper Capsicum spp.
Potato Solanum tuberosum
Radish Raphanus sativus
Rye Secale cereale
Sugar beet Beta vulgaris
Sunflower Helianthus annuus
Sweet potato Impomea batatas
Watermelon Citrullus lanatas
the pollen of the male is sterile, compels the plant to
receive pollen from different flowers. Similarly, a condition
called heterostyly is one in which significant
difference in the lengths of the stamen and pistil makes
it less likely for self-pollination to occur, and thereby
promotes cross-pollination. Cross-pollinated species
depend on agents of pollination, especially wind and
insects. A partial list of cross-pollinated species is presented
in Table 4.3.
Genetic and breeding implications of cross-pollination
The genotype of the sporophytic generation is heterozygous
while the genotypes of gametes of a single
plant are all different. The genetic structure of a crosspollinated
species is characterized by heterozygosity.
Self-incompatibility occurs in such species. Unlike selfpollinated
species in which new gene combinations are
prohibited, cross-pollinated species share a wide gene
pool from which new combinations are created to form
the next generation. Furthermore, when cross-pollinated
species are selfed, they suffer inbreeding depression.
Deleterious recessive alleles that were suppressed because
of heterozygous advantage have opportunities to be