Biology Reference
In-Depth Information
tapetal chamber keeping the young microspores in close proximity may contrib-
ute to this wall maintenance. Surprisingly, the young microspores are apparently
separated from their sibling cells allowing some free movement indicated by the
strange 180° rotation of the pollen aperture sites. Thus, those aperture sites that
originally looked outwards rotate inward to face each other. A similar rotation
has been reported previously in other Annonaceae [A. glabra and A. montana
[22] and Cymbopetalum [23], and also in species of the Poaceae [51]. This distal-
proximal microspore polarity transition in development contrasts with the evolu-
tionary shift from a proximal to a distal aperture that has been long regarded as
one of the major evolutionary innovations in seed plants [52]. Proximal apertures
predominate in the spores of mosses, lycophytes and ferns while distal apertures
are more common in extant seed plants including gymnosperms, cycads and ear-
ly-divergent angiosperms [52]. In fact, species in the Annonaceae with monad
pollen are reported to have distal apertures [see [19] for review]. However, a com-
plete study of 25 Annonaceae genera with species that release aggregated pollen
showed proximal apertures [16] and, consequently, the distal-proximal transition
observed in pollen development of A. cherimola and other Annonaceae [22,23]
could represent a widespread situation in this basal family.
Another reason proposed for this permanent binding of pollen in groups of
four could be a failure in the synthesis of the callose layer during microspore sepa-
ration in the tetrad [8]. However, the results shown in this work in A. cherimola
indicate that callose is layered following the standard pattern and vanishes later,
after meiosis is completed, similar to the way it occurs in Arabidopsis quartet
mutants, in which callose dissolution proceeds normally [53]. However, the use
of antibodies against callose showed that callose remains for a while in the area
where pollen apertures will form hampering the layering of sporopollenin. Callose
remnants in this area have also been reported in other Annonaceae and it has been
suggested that these remnants pull the pollen grains to undergo the 180° turning
[22,23]. In the formation of the pollen wall, callose dissolution occurs concomi-
tantly with the layering of the exine [54] and the formation of the pollen aperture
is related to endoplasmic reticulum blocking the deposition of primexine [3]. The
callose remnant at the pollen aperture sites has not been investigated in detail in
other species and, given the high conservation of pollen ontogeny in angiosperms,
this is a topic worthy of a detailed study. Interestingly, in an Arabidopsis mutant
lacking the gene responsible for callose synthesis, pollen develops unusual pore
structures [55].
Further binding at the aperture sites could follow this initial adhesion process
through the observed joint deposition of pollenkit that has also been reported in
other species [4]. Thus, two key processes could contribute to holding together
