Biology Reference
In-Depth Information
the four pollen grains in A. cherimola, the confined space that permits the delay
in the dissolution of the PMC wall and the tapetal chamber and pollen rota-
tion that allows the adhesion of the sticky proximal faces by the formation of
small pectin bridges. Later, the join deposition of sporopollenin would further
strengthen this initial binding.
Biological Significance of the Pollen Dispersal Unit
A failure or delay in the dissolution of the PMC wall and tapetal chamber
appears to be a critical step, resulting in the continued proximity of the four
microspores produced by meiosis of a single PMC. However, this phenotype
could also result from failure in the different enzymes that dissolve the PMC
wall. The distribution of this character, together with the information pro-
vided by Arabidopsis mutants, shows that this has occurred independently
several times during evolution, suggesting that it must provide some evolu-
tionary advantages [8].
The adaptive advantages derived from aggregated pollen have been reviewed
recently [6]. The release of aggregated pollen in insect pollinated species could
increase pollination efficiency, since more pollen grains could be transferred in a
single pollinator visit and, in this sense, a correlation between pollen tetrads and
polyads with a high number of ovules per flower has been shown in a survey of
the Annonaceae [17]. The release of aggregated pollen is more advantageous in
situations where pollinators are infrequent [6] and in situations of short pollen
viability and pollen transport periods. A short pollen viability period has been
reported in A. cherimola, [47,56] and a short pollen transport episode is com-
mon in several Annonaceae [57] and in other beetle pollinated species of early
divergent angiosperm lineages [58].
An additional possible benefit of aggregated pollen is protection against
desiccation and entry of pathogens through the thin walls of the pollen aper-
ture sites. Pollen grouped in dyads, tetrads or polyads show a strong proximal
reduction of the exine in Annonaceae [59]. A. cherimola pollen is inapertu-
rate and germinates in the proximal face, showing a large area of unprotected
intine [47,60]. More evolutionarily recent species present a colpus that, in
dehydrated pollen, is just a narrow slit protected by loose pollenkit. Only
upon hydration, when the pollen faces a wet surface on the stigma, this slit
swells developing a wider colpus through which the pollen tube protrudes
[61]. Inaperturate pollen does not have this protection from desiccation and
the development of inward facing intines may play a role in protecting pollen
against desiccation.
