Pollination Drop in Relation to Cone Morphology in Podocarpaceae: A Novel Reproductive Mechanism

Abstract

Observation of ovulate cones at the time of pollination in the southern coniferous family Podocarpaceae demonstrates a distinctive method of pollen capture, involving an extended pollination drop. Ovules in all genera of the family are orthotropous and single within the axil of each fertile bract. In Microstrobus and Phyllocladus ovules are erect (i.e., the micropyle directed away from the cone axis) and are not associated with an ovule‐supporting structure (epimatium). Pollen in these two genera must land directly on the pollination drop in the way usual for gymnosperms, as observed in Phyllocladus. In all other genera, the ovule is inverted (i.e., the micropyle is directed toward the cone axis) and supported by a specialized ovule‐supporting structure (epimatium). In Saxegothaea there is no pollination drop and gametes are delivered to the ovule by pollen tube growth. Pollination drops were observed in seven of the remaining genera. In these genera the drop extends over the adjacent bract surface or cone axis and can retain pollen that has arrived prior to drop secretion (“pollen scavenging”). The pollen floats upward into the micropylar cavity. The configuration of the cone in other genera in which a pollination drop has not yet been observed directly suggests that pollen scavenging is general within the family and may increase pollination efficiency by extending pollination in space and time. Increased pollination efficiency may relate to the reduction of ovule number in each cone, often to one in many genera, a derived condition. A biological perspective suggests that animal dispersal of large seeds may be the ultimate adaptive driving force that has generated the need for greater pollination efficiency.

Ovular secretions as part of pollination mechanisms in conifers

“…Degeneration of the nucellus varies among genera. Tissue breakdown is quite extensive in some genera, such as Cephalotaxus [10], Cupressus [68], and Podocarpus [71]. In others, breakdown is restricted to the uppermost layer, as has been shown in studies of Sequoiadendron [64] and Phyllocladus [68].…”

Generalist flowers and phytophagous beetles in two tropical canopy trees: resources for multitudes

“…In a few other taxa, such as Larix and Pseudotsuga (Doyle & O'Leary, 1935;Tomlinson & al., 1991;Owens & al., 1998), the production of pollination drops is delayed until after pollen lands on a dry micropylar surface but occurs prior to fertilization. In extant gymnosperms, pollination drops are timed for maximum reception of saccate or non-saccate pollen by variously oriented ovules (Gelbart & Aderkas, 2002).…”

Flotation preferentially selects saccate pollen during conifer pollination

“…, 1996; Schwendemann et al. , 2007), they may instead function primarily as floats that aid in the movement of pollen into the ovule during pollen capture (Doyle & O’Leary, 1935; Doyle, 1945; Tomlinson et al. , 1991; Tomlinson, 1994).…”

“…375 species in the conifer families Pinaceae and Podocarpaceae) share a correlated suite of ovular characteristics, including ovules that open downwards with respect to gravity at the time of pollination and ovules that produce aqueous secretions or utilize rainwater as ‘pollination drops’– a transient drop of liquid on the tip of the gymnosperm ovule (Fig. 1a–d; Doyle & O’Leary, 1935; Doyle, 1945; Tomlinson et al. , 1991; Tomlinson, 1994; Takaso & Owens, 1995; Owens et al.…”