The Questions of Developmental Biology

*A small weed in the mustard family, Arabidopsis is used as a model system because of its very
small genome.
Pollination
Pollination refers to the landing and subsequent germination of the pollen on the stigma. Hence it
involves an interaction between the gametophytic generation of the male (the pollen) and the
sporophytic generation of the female (the stigmatic surface of the carpel). Pollination can occur
within a single flower (self-fertilization), or pollen can land on a different flower on the same or a
different plant. About 96% of flowering plant species produce male and female gametophytes on
the same plant. However, about 25% of these produce two different types of flowers on the same
plant, rather than perfect flowers containing both male and female gametophytes. Staminate
flowers lack carpels, while carpellate flowers lack stamens. Maize plants, for example, have
staminate (tassel) and carpellate (ear) flowers on the same plant. Such species are considered to
be monoecious (Greek mono, "one"; oecos, "house"). The remaining 4% of species (e.g.,
willows) produce staminate and carpellate flowers on separate plants . These species are
considered to be dioecious ("two houses"). Only a few plant species have true sex chromosomes.
The terms "male" and "female" are most correctly applied only to the gametophyte generation of
heterosporous plants, not to the sporophyte (Cruden and Lloyd 1995).
The arrival of a viable pollen grain on a receptive stigma does not guarantee fertilization.
Interspecific incompatibility refers to the failure of pollen from one species to germinate and/or
grow on the stigma of another species (for a review, see Taylor 1996). Intraspecific
incompatibility is incompatibility that occurs within a species. Self-incompatibility
incompatibility between the pollen and the stigmas of the same individual is an example of
intraspecific incompatibility. Self-incompatibility blocks fertilization between two genetically
similar gametes, increasing the probability of new gene combinations by promoting outcrossing
(pollination by a different individual of the same species). Groups of closely related plants can
contain a mix of self-compatible and self-incompatible species.
Several different self-incompatibility systems have evolved (Figure 20.9). Recognition of self
depends on the multiallelic self-incompatibility (S) locus (Nasrallah et al. 1994; Dodds et al.
1996; Gaude and McCormick 1999). Gametophytic self-incompatibility occurs when the S allele
of the pollen grain matches either of the S alleles of the stigma (remember that the stigma is part
of the diploid sporophyte generation, which has two S alleles). In this case, the pollen tube begins
developing, but stops before reaching the micropyle. Sporophytic self-incompatibility occurs
when one of the two S alleles of the pollen-producing sporophyte (not the gametophyte) matches
one of the S alleles of the stigma. Most likely, sporophyte contributions to the exine are
responsible.
The S locus consists of several physically linked genes that regulate recognition and rejection of
pollen. An S gene has been cloned that codes for an RNase called S RNase, which is sufficient, in
the gametophytically self-incompatible petunia pistil, to recognize and reject self-pollen (Lee et
al. 1994). The pollen component recognized is most likely a different gene in the S locus, but has
not yet been identified in either gametophytically or sporophytically self-incompatible plants. In
sporophytic self-incompatibility, a ligand on the pollen is thought to bind to a membrane-bound
kinase receptor in the stigma that starts a signaling process leading to pollen rejection. The
mechanism of pollen degradation is unclear, but appears to be highly specific.