Biology Reference
In-Depth Information
Angiosperms pollen is most commonly released as single pollen grains or
monads [4] which represent the basic angiosperm pollen-unit. Dehiscence of ag-
gregated pollen (mostly in groups of four) is considered a recent apomorphic
characteristic [5,6] that has arisen independently several times during evolution
primarily in animal-pollinated taxa although, in some cases, monads may have
evolved secondarily from groups of four grains [6]. Pollen release as tetrads has
been reported in some or all members of 55 different angiosperm families and also
in some pteridophytes [7]. Blackmore and Crane (1988) [8] put forward that the
maintenance of pollen tetrads could be the result of relatively minor ontogenetic
changes and, consequently, this could be an excellent example of convergence in
situations where the release of pollen as tetrads is an effective reproductive strat-
egy. Interestingly, the dissemination of pollen as tetrads has also been reported in
the quartet mutants of Arabidopsis [9,10].
Annonaceae, included in the order Magnoliales, is the largest family within
the basal angiosperm Magnoliid clade [11,12]. Due to its phylogenetic position
among the basal angiosperms, the family has been the object of considerable in-
terest from a taxonomic and phylogenetic point of view [13-15] and a number
of studies have focused on pollen morphology [16-20]. Although most genera
of the Annonaceae produce solitary pollen at maturity, in several species of the
family pollen is released aggregated in groups of four or in polyads [17]. Recent
studies on the mechanism of pollen cohesion in this family have been performed
in species of the genera Pseuduvaria [21], Annona and Cymbopetalum [22,23].
Pollen cohesion in these species is generally acalymmate (four pollen grains are
grouped only by partial fusion) with simple cohesion [21]. But these studies show
differences in cohesion mechanisms; thus, while pollen grains in Pseuduvaria are
connected by wall bridges (crosswall cohesion), involving both the exine and the
intine, in A. glabra, A. montana and Cymbopetalum cohesion is achieved through
a mass of callose-cellulose. Evolutionary transitions in flowering plant reproduc-
tion are proving to have a clear potential in plant evolutionary biology [24], and
the need for more detailed ontogenetic studies in the family has been put forward
[22]. Indeed the fact of being the largest family among basal angiosperms, to-
gether with the puzzling connection mechanisms so far described in the different
species examined, provide an excellent opportunity to investigate the ontogeny of
pollen development and its evolutionary implications.
In this work, pollen development is characterized in A. cherimola, one of the
species in the Annonaceae where pollen is shed aggregated in groups of four, pay-
ing special attention to the events close to pollen formation and retention of the
individual pollen grains together, observed by immunolocalization of different
wall components. Results are discussed in relation to the shedding of pollen in
groups of four in other species and how this event may have occurred and settled
during evolution.
