APERTURE ONTOGENY IN THE PROTEACEAE GREVILLEA ROSMARINIFOLIA pp. 349-354
Authors: (Béatrice Albert, Sophie Nadot, Univ Paris-Sud, Ecologie Systématique et Evolution, Orsay, France)
Abstract: The apertures are the first elements of the pollen wall that are established during pollen ontogeny, since they are already visible in the late tetrad stage before any other feature of the ornamentation. They are established during microsporogenesis (male meiosis), the earliest step in pollen development. Apertures are functionally important for the pollen grain since they are implicated in pollen grain survival and reproductive success (Dajoz, Till-Bottraud, and Gouyon, 1993; Till-Bottraud et al., 1994; Till-Bottraud et al., 1999; Till-Bottraud et al., 2001). A wide range of variation in aperture pattern is observed throughout angiosperms (Erdtman, 1952) but overall the number of apertures is stable within the large taxonomic subdivisions. The emergence of tricolpate pollen is thought to be involved in the success of eudicots (Furness and Rudall, 2004) of which is it the main synapomorphy. Elsewhere (monocots and basal angiosperms), pollen grains are predominantly mono-aperturate. Aperture pattern has been suggested to be linked to features of microsporogenesis such as the type of cytokinesis (successive vs. simultaneous), cell plate formation (centrifugal vs. centripetal, relatively to the centre of the tetrad, or to the centre of each of the cleavage planes), and tetrad shape (tetrahedral, tetragonal or /rhomboidal, plus other less common shapes) (Ressayre et al., 2002a; Blackmore et al., 2007). The difference in pollen aperture pattern between monocots and eudicots is associated with divergences in the features of microsporogenesis. For example, microsporogenesis can vary among genera or even species within the monocots whereas it is usually highly conserved within the eudicots (Furness and Rudall, 1999; Furness, Rudall, and Sampson, 2002; Penet et al., 2005; Ressayre et al., 2005; Nadot et al., 2006; Sannier et al., 2006 ; Sannier et al., 2007). Intersporal wall formation by centrifugal cell plates, associated with successive cytokinesis and resulting in tetragonal tetrads is common in monocots but has never been described in eudicots. In both monocots and eudicots, inaperturate pollen can be obtained through various pathways of microsporogenesis (Nadot et al., 2006; Furness, 2007; Albert, Gouyon, and Ressayre, 2009; Lopes Pereira Nunes, Bona, and Ike Coan, 2009) suggesting that constraints on microsporogenesis may be released in the absence of apertures.