Reproduction in Seagrasses: Pollen Development in Thalassia hemprichii, Halophila stipulacea and Thalassodendron ciliatum

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Abstract
The general features of pollen morphogenesis in three marine monocotyledons, Thalassia hemprichii, Halophila stipulacea and Thalassodendron ciliatum, are described in this paper. Thalassia disperses spherical trinucleate pollen grains. In this genus simultaneous cytokinesis generally produces an isobilateral tetrad of microspores, but linear and T-shaped configurations also occur, together with configurations intermediate between isobilateral and T-shaped. Partitioning is followed by a phase of cellular degeneration affecting one or two, never more, members of the tetrad. Subsequent development of the surviving, functional microspores does not differ essentially from the pattern of morphogenesis in terrestrial flowering plants. Halophila disperses strings of four reniform trinucleate pollen grains contained in a mucilaginous moniliform tube. These arise by successive transverse partitioning of an elongate mother cell and the linear unit so formed is maintained throughout pollen development. The tetrad tube substance originates in the tapetal periplasmodium and deposition begins soon after meiosis. Thalassodendron disperses filiform trinucleate pollen grains. The characteristic form of the pollen in this genus is attained during post-meiotic growth and differentiation, as in other genera belonging to the same family. This contrasts with the situation in seagrasses belonging to the Zosteraceae where the filiform shape is established before meiosis. Precocious division of the microspore nucleus in Thalassodendron launches the binucleate pollen phase soon after the spores separate from the tetrad. The division precedes the vacuolate period; again, this is a feature of the family. In Thalassia the tapetal periplasmodium is progressively transformed into thecal slime. In Thalassodendron and Halophila the periplasmodial residue forms a superficial coating on the pollen wall and tetrad tube. These products could be implicated in attachment and recognition of the pollen at the stigma surface.