Comparative morphological, anatomical and embryological studies of Prumnopitys taxifolia and P. ferruginea (Podocarpaceae), and the hydrodynamics of their saccate pollen grains
Prumnopitys taxifolia (Banks et Sol. ex D. Don) de Laub. (matai) and P. ferruginea (D. Don) de Laub. (miro) are two New Zealand podocarps with morphologically very different reproductive structures. In P. taxifolia the pollen cones are sessile on spicate fertile branches, whereas in P. ferruginea they are solitary on short scaly peduncles. Likewise, in P. taxifolia the ovulate strobilus is an attenuated spicate structure bearing globose seeds, whereas in P. ferruginea the reduced ovulate strobilus bears a dorsi-ventrally flattened, usually solitary seed on a slender scaly peduncle. Such differences throw doubt on the validity of the current delimitations of the genus Prumnopitys. A comparative embryological study of these two species was done in parallel with a morphological and anatomical study of the reproductive structures to fill gaps in their known reproductive life cycles, and to determine whether there are any differences in the male and female cones or in their embryogeny that are of taxonomic significance at the generic level. In addition the hydrodynamic properties of their saccate pollen grains were compared with that of the pollen of several other conifers and the spores of three ferns and one lycopod.
Morphology and Embryology
Embryologically these two species are shown to be more similar to each other and to the Chilean Prumnopitys andina than to other podocarp genera. For instance, in both species: the long tapered archegonia have prominent neck cells with a thickened cap overlying them; five free nuclear mitoses occur before cell formation in the proembryo; meiosis can result in both linear and non-linear tetrads of megaspores; the microgametophyte typically forms 7 or 8 prothallial cells before dispersal.
However, new morphological and anatomical differences have emerged from this study. In the pollen cones of P. taxifolia, the microsporangia are free with a transverse stomium located close to the cone axis; in P. ferruginea they are fused with the transverse stomium located basally. In P. taxifolia ovules, a tanniniferous layer develops in the epimatium and not in the integument, and fusion of epimatium/integument/nucellus is greatest dorsi-ventrally; in P. ferruginea ovules a prominent tanniniferous layer develops only in the integument, and fusion of epimatium/integument/nucellus is greatest laterally. Furthermore, observations of insect predation point to biochemical differences between P. taxifolia and P. ferruginea. An undescribed species of gall midge appears to be specialized exclusively for the P. taxifolia life cycle, and several other insect larvae prey on the developing and mature seeds, whereas the ovules/seeds of P. ferruginea appear to have almost no insect predators. Morphologically, anatomically and biochemically, P. taxifolia and P. ferruginea may therefore be sufficiently different to warrant being two separate genera; comparable differences have been used to distinguish other genera in the Podocarpaceae.
Since embryogeny is likely to be under less evolutionary pressure than morphology and anatomy, and since P. taxifolia (along with P. andina) is regarded by some as basal in the conifers, with P. ferruginea less so, their embryological similarity does not necessarily indicate close relationship, only that they are closer to each other than to the other podocarps. Although their morphological and anatomical differences may be of taxonomic significance, a better understanding of the other species currently in Prumnopitys is necessary before the relationships within the genus can be clarified.
Despite the morphological differences in their ovulate cones, P. taxifolia and P. ferruginea have very similar pollination mechanisms involving an inverted micropyle, a pollination drop and saccate pollen. Saccate grains have sometimes been referred to as 'non-wettable' due to their buoyant properties, while non-saccate pollen grains have been described as 'wettable'. The hydrodynamic properties of saccate pollen grains of seven podocarp species in five genera, Dacrydium Sol. ex G. Forst., Dacrycarpus (Endl.) de Laub., Manoao Molloy, Podocarpus L'Hér. ex pers. and Prumnopitys Phil. have been tested in water, together with saccate and non-saccate pollen of four other conifer genera, Cedrus Trew (Pinaceae), Cephalotaxus Siebold & Zucc. ex Endl. (Cephalotaxaceae), Cupressus L. (Cupressaceae) and Phyllocladus Rich. ex Mirb. (Phyllocladaceae), and spores of three fern species and one lycopod species. All four spore types studied were non-wettable, whereas the bisaccate and trisaccate pollen types, like all the other conifer pollen types, were wettable, enabling the grains to cross the surface tension barrier of water. Once past this barrier, grain behaviour was governed by presence or absence of sacci. Non-saccate and vestigially saccate grains sank, whereas saccate grains behaved like air bubbles, floating up to the highest point. In addition, the grains were observed to float in water with sacci uppermost, consistent with the suggestion that distally-placed sacci serve to orientate the germinal furrow of the pollen grain towards the nucellus of an inverted ovule. Observations of pollen grains in the pollen chambers of naturally pollinated Prumnopitys ovules confirmed this. The combination of buoyancy and wettability in saccate pollen has implications for the efficiency of the typical podocarp pollination mechanism.
Since saccate pollen, inverted ovules and pollination drops are also characteristic of the Pinaceae, the similarities in the pollination mechanisms and the pollen grains of P. taxifolia and P. ferruginea have no taxonomic significance at the generic level.