Pollen Morphology and Ultrastructure of the Cabombaceae: Correlations With Pollination Biology

Abstract

Of all species comprising the two genera of the Cabombaceae, only Brasenia schreberi J. F. Gmel. and Cabomba caroliniana Gray have been critically investigated with regard to their pollination biology. Brasenia schreberi has been shown to be anemophilous, while C. caroliniana has an entomophilous (myophilous) pollination syndrome. In the present paper, a number of pollen and pollen‐related characters, including pollen size, shape, quantity, terminal settling velocity, pollen‐ovule ratios, and overall exine architecture of B. schreberi and C. caroliniana are evaluated. Pollen from both species is elliptic, monosulcate, and has a tectate‐columellate sporoderm with supratectal surface ornamentation. Grains of B. schreberi are small, produced in copious amounts, and settle relatively slowly. Flowers of this species have large pollen‐ovule ratios. The exine of B. schreberi pollen is scabrate, relatively thin, has a uniformly thick sexine composed of a two‐zoned (homogeneous/granular) tectum and distinct columellae, and a homogeneous nexine. Pollen of C. caroliniana is relatively large, produced in small quantities, and has a rapid terminal settling velocity. Flowers exhibit small pollen‐ovule ratios. Exine organization of C. caroliniana pollen is typically two times thicker than that of B. schreberi; ornamentation is striate. Nonapertural sexine regions have a thick tectum and well‐defined columellae, with both sexine components traversed by a dense system of channels. The nexine is relatively thin. All of the palynological characters examined correlate well with the anemophilous and entomophilous syndromes of B. schreberi and C. caroliniana, respectively. Moreover, several other parameters of exine ultrastructure from each species exhibit positive correlations with the respective pollination mechanisms, including: tectum thickness, columellae diameter, tectum‐nexine ratios, and the consistency, distribution, and total amount of pollenkitt present. Overall exine ultrastructure is also discussed from a historical perspective as well as with respect to its phylogenetic significance.

Pollen grain numbers, ovule numbers and pollen-ovule ratios in Caryophylloideae: correlation with breeding system, pollination, life form, style number, and sexual system

“…Compared with other Caryophylloideae, both species have extremely large stigma areas and, due to the relatively long calyx length of these species, the pollen load is deposited mainly on the head and the proboscis of the pollinators (A. Jürgens, unpublished observation). According to Cruden (2000), stigma area relative to the pollen-bearing area directly influences the likelihood of a pollen grain reaching the stigma, and species with large stigma areas relative to pollen-bearing areas often show quite low P/Os (Mehrhoff 1983;Osborn et al 1991;Cruden 2000). There is, however, an alternative explanation for the low P/Os in S. dioica and S. latifolia.…”

Pollen morphology ofEphedra(Gnetales) and its evolutionary implications

“…Consequently, pollen characters have shown to be informative in studies of plant evolution and for resolving phylogenies (e.g. Doyle & Le Thomas 1994;Doyle & Endress 2000;Sauquet & Le Thomas 2003), for calibrating molecular dating analyses (Thornhill et al 2012), as well as for studying plant reproductive biology (Ferguson & Skvarla 1982;Grayum 1986;Osborn et al 1991;Bolinder et al 2015). In addition, fossil pollen data are also frequently used for reconstructing past vegetation types and for inferring paleoclimates (Hoorn et al 2012).…”

“…It is further clear that there has been a trend towards reduction of the number of plicae during the course of evolution in Ephedra ( Figure 14A). The morphology and structure of the exine are known to have substantial implications for the pollination biology of plants (Ferguson & Skvarla 1982;Grayum 1986;Osborn et al 1991;Bolinder et al 2015). For entomophilous pollen to be successfully transported from the microsporangiate structures to the ovulate structures (specifically in this case, to the pollination drop), the pollen needs to adhere to an insect vector.…”

Aerodynamics and pollen ultrastructure in Ephedra

“…In contrast, pollen grains of insect-pollinated taxa generally have low dispersability because of their higher U t . The U t of insect-pollinated taxa is rarely measured but, for example, pollen of the insectpollinated species Cabomba caroliniana A.Gray has a U t of 19.5 cm·s −1 ( Osborn et al, 1991 ). Although the difference in U t between the various species of Ephedra are less dramatic compared with that between P. sylvestris and C. caroliniana , we nevertheless judge these differences apparent and signifi cant, as well as congruent with previous studies of the pollination biology of this genus.…”