Principles of Plant Genetics and Breeding

488 CHAPTER 28
may be used to develop homozygous diploid inbred
lines for hybrid production. They can also be used to
convert inbred lines with male fertility to male-sterile
cytoplasm. Tetraploid corn was shown to have gigas
features (e.g., regarding leaves, tassels, ears, kernel size)
but with reduced fertility. Tetraploid yellow corn also
produces about 40% higher carotenoid pigment content
than the diploid parent. Barbara McClintock conducted
extensive cytological work on maize. She developed a
complete primary trisomic series, using triploids, of
which only trisomy 5, 7, and 8 can be distinguished and
characterized phenotypically. Primary trisomics have
been used to assign genes to specific chromosomes.
Intergeneric crosses between cultivated corn and
related genera, teosinte (Euchlaena spp.) and gammagrass
(Tripsacum spp.), have been accomplished. The
success is more common with annual teosinte (Z.
mexicana) in which the annual strains (“Chalco”,
“Durango”, “Florida” varieties) readily cross with cultivated
corn. Tripsacum may be diploid (2n = 36) or
tetraploid (2n = 72). It is a potential source of resistance
to many diseases and insect pests of corn. The F 1 is
backcrossed to maize to remove all the Tripsacum
chromosomes, leaving a plant that exhibits a pure maize
phenotype. However, yield is reduced and so is agronomic
suitability, making this introgression, overall, less
attractive at the present time.
In addition to the 10 A-chromosomes, corn may have
additional chromosomes, called B-chromosomes or
supernumerary chromosomes, that are genetically inert
(have no known impact on normal growth).
Genetics
Corn is one of the plants that has been genetically widely
studied. Hundreds of mutations have been identified in
corn that impact traits such as plant height, endosperm
characteristics, plant colors, insect resistance, disease
resistance, stalk strength, and many other traits. Some of
the significant genetic effects are discussed here.
Xenia
Xenia is the immediate effect of pollen on the developing
kernel. It may be observed when two varieties differing
in a single visible endosperm trait are crossed. Xenia
occurs when the trait difference is conditioned by a
dominant gene present in the pollen. However, when
dominance is incomplete, xenia will occur when either
variety is the pollen parent. Xenia is important because
endosperm characteristics distinguish some of the major
corn groups. For example, starchy endosperm is dominant
over sugary (sweet) and waxy endosperm. A cross
of starchy × sugary exhibits xenia. Similarly, a cross of
shrunken × non-shrunken endosperm, waxy × non-waxy
endosperm, purple × colorless aleurone, and yellow ×
white (colorless) endosperm, all exhibit xenia.
Whereas xenia may result from simple dominance
gene action, the effect is different in some instances. In
the cross of flinty × floury endosperm, the F 1 is flinty
(FFf ). However, the reciprocal cross of floury × flinty
endosperm produces an F 1 with floury endosperm (ffF),
indicating the ineffectiveness of the dominant allele (F)
to overcome the double recessive (rr) floury genes.
Similarly, xenia in aleurone color depends on the combined
action of five dominant genes (designated A1, A2,
C, R, and Pr).
Chlorophyll varieties
Numerous leaf color abnormalities that affect the corn
plant in both the seedling and mature stages have
been identified. Chlorophyll-deficient mutations cause
a variety of leaf colors such as albino, virescent, and
luteus (yellow). Mature plants exhibiting golden, greenstripped,
and other leaf patterns are known.
Transposable elements
Genomes are relatively static. However, they evolve,
albeit slowly, by either acquiring new sequences or
rearranging existing sequences. Genomes acquire new
sequences either by mutation of existing sequences or
through introduction (e.g., by vectors, hybridization).
Rearrangements occur by certain processes, chiefly
genetic recombination and transposable genetic elements
(see Chapter 12).
The ancient allotetraploid origins and the presence
of large numbers of transposable elements makes the
maize genome complex. However, it is these very features
that make the maize plant suitable for functional
genomics studies. The number and variety of transposable
elements facilitate insertional mutagenesis projects.
Further, its allotetraploid-based gene redundancy allows
scientists to characterize mutants that may be lethal in a
diploid species.
Cytoplasmic male sterility (CMS)
Male-sterility genes are among the most important
mutations in corn from the standpoint of breeding.