A MORPHOGENETIC MODEL ACCOUNTING FOR POLLEN APERTURE PATTERN IN FLOWERING PLANTS

Pollen grains are embedded in an extremely resistant wall. Apertures a re well defined places where the pollen wall is reduced or absent that permit pollen tube germination. Pollen grains are produced by meiosis and aperture number definition appears to be linked with the partitio n that follows meiosis and leads to the formation of a tetrad of four haploid microspores. In dicotyledonous plants, meiosis is simultaneous which means that cytokinesis occurs once the two nuclear divisions ar e completed. A syncitium with the four nuclei stemming from meiosis is formed and cytokinesis isolates simultaneously the four products of m eiosis. We propose a theoretical morphogenetic model which takes into account part of the features of the ontogeny of the pollen grains. The nuclei are considered as attractors acting upon a morphogenetic subst ance distributed within the cytoplasm of the dividing cell. This leads to a partition of the volume of the cell in four domains that is simi lar to the observations of cytokinesis in the studied species. The mos t widespread pattern of aperture distribution in dicotyledonous plants (three apertures equidistributed on the pollen grain equator) can be explained by bipolar interactions between nuclei stemming from the sec ond meiotic division, and observed variations on these patterns by dis turbances of these interactions. In numerous plant species, several po llen grains differing in aperture number are produced by a single indi vidual. The distribution of the different morphs within tetrads indica tes that the four daughter cells can have different aperture number. T he model provides an explanation for the duplication of one of the ape rtures of a three-aperturate pollen grain leading to a four-aperturate one and in parallel it gives an explanation for how heterogeneous tet rads can be formed. (C) 1998 Academic Press