gameteogenesis(the formation of gametes) orfertilization, geneticists have utilized mutant
populations of a superb experimental model plant calledArabidopsis thaliana. The genome
ofArabidopsisis fully sequenced, and many different mutant populations containing a loss
of function in individual genes are available. The first mutant collections were most often
composed of plants containing random, single-base-pair mutations, or T-DNA insertions.
Both types of mutant collections can be screened, and mutants identified on the basis of
the phenotype. For example, mutants defective in a gene required to form a female or
male gamete will give rise to mature plants with low fertility. Low fertility can be somewhat
easily scored in a random mutant population by looking for low seed set. InArabidopsis,
the seeds are produced within small, elongated fruits calledsiliques. Finding a plant with
fewer siliques, or empty siliques, is an indication that a mutant has lost the function in a
gene required for gamete development. One can determine which gamete has been affected
by examining the appearance of both female and male gametes from the candidate mutant
plant. For example, if pollen grains appear normal and germinate pollen tubes in vitro, then
most likely, the defect isnotin male gametophyte development. The scientist would then
examine the appearance of the female gametophyte within the flower. Outcrosses of the
candidate mutant pollen to a wild-type pistil, and the reverse outcross (candidate mutant
female to wild-type male cross), can also be important in determining which gamete is
defective (Wilson and Yang 2004; Boavida et al. 2005).
Using such screens and outcrosses, geneticists have isolated several genes required for
pollen and ovule development. Inpopmutants, for example, theexine layerof the pollen
grain does not develop properly, resulting in altered hydration of pollen grains. Without
normal hydration, the pollen tube guidance is not normal, and fertilization is greatly
lowered. These mutants point to the idea that structural components of the pollen grain
itself are important for male fertility.
Female sterile mutants have also led to the identification of genes required in female
gametophytic development. ANT, BEL1, SIN1, and ATS gene products were each
Figure 4.2.Gametogenesis. Schematics of (a) anArabidopsisflower with the floral organs identified;
(b) a cross section through the male organs (anther, filament) showing the site of male gamete for-
mation; (c) the female ovule contained within the carpels of the flower showing the site of female
gamete development. [Reprinted from Wilson and Yang C (2004), with permission from the
Society for Reproduction and Fertility.]
86 PLANT DEVELOPMENT AND PHYSIOLOGY