Unique adaptations for submarine pollination in seagrasses

Pollen in water of unstable salinity: Evolution and function of dynamic apertures in monocot aquatics

PREMISE

The sporoderm of seed-plant pollen grains typically has apertures in which the outer sporopollenin-bearing layer is relatively sparse. The apertures allow regulation of the internal volume of the pollen grain during desiccation and rehydration (harmomegathy) and also serve as sites of pollen germination. A small fraction of angiosperms undergo pollination in water or at the water surface, where desiccation is unlikely. Their pollen grains commonly lack apertures, though with some notable exceptions. We tested a hypothesis that in some angiosperm aquatics that inhabit water of unstable salinity, the pollen apertures accommodate osmotic effects that occur during pollination in such conditions.

METHODS

Pollen grains of the tepaloid clade of the monocot order Alismatales, which contains ecologically diverse aquatic and marshy plants, were examined using light microscopy and scanning electron microscopy. We used Ruppia as a model to test pollen grain response in water of various salinities. Pollen aperture evolution was also analyzed using molecular tree topologies.

RESULTS

Phylogenetic optimizations demonstrated an evolutionary loss and two subsequent regains of the aperturate condition in the tepaloid clade of Alismatales. Both of the taxa that have reverted to aperturate pollen (Ruppia, Ruppiaceae; Althenia, Potamogetonaceae) are adapted to changeable water salinity. Direct experiments with Ruppia showed that the pollen apertures have a role in a harmomegathic response to differences in water salinity.

CONCLUSIONS

Our results showed that the inferred regain of pollen apertures represents an adaptation to changeable water salinity. We invoke a loss-and-regain scenario, prompting questions that are testable using developmental genetics and plant physiology.

Tapetum character states: analytical keys for tapetum types and activities

The different types of tapetum found in the spermatophyta are described, along with associated characters. The characters (taken singly, pairwise, or in multiple combinations) are (i) tapetum types; (ii) cell walls, tapetum types, and loculus; (iii) tapetal cells individually, tapetum types, and loculus; (iv) number of pollen grains enveloped by tapetal cells and type of pollen dispersing unit; (v) cell types and tapetum types; (vi) number of nuclei per cell and tapetum type; (vii) cycles of hyperactivity; (viii) exine formation; (ix) orbicles; (x) peritapetal membrane; (xi) plastid differentiation; (xii) stage of pollen development in which tapetal cells degenerate and type of pollen coat; (xiii) storage vacuoles; (xiv) sporophytic proteins; and (xv) devices of tapetal origin responsible for compound pollen formation and pollination. Examples are given and an analytical key of structural and functional diversity is provided as a helpful approach to the study of the tapetum. Key words: tapetum types, activities, pollen dispersing units.