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Breeding Systems in Gymnosperms -- Relative Benefits and Costs of Being Monoecious or Dioecious

To start, a couple of definitions:

Dioecious plants are either male or female. Each individual plant, when it produces reproductive structures, produces only male or female structures.

Monoecious plants are both male and female. In gymnosperms, this means that the plant bears both male and female reproductive structures. Generally, the male and female reproductive structures are located in different parts of the plant, and/or become fertile at different times, to minimize the risk that the plant will reproduce with itself (this would largely negate the whole point of sexual reproduction, which is gene recombination). For instance, most monoecious conifers bear seed cones in the upper crown and pollen cones in the lower crown.

The question of which gymnosperms are monoecious and which dioecious was treated in encyclopedic manner by Givnish (1980), who summarized his work as follows (paraphrased):

Animal-dispersed gymnosperms are usually dioecious and wind-dispersed gymnosperms are usually monoecious. This correlation between breeding system and dispersal syndrome confers a disproportionate advantage in seed dispersal for plants with exceptionally large seed crops. Dioecy should be rare in herbs and shrubs; in plants whose flowers or floral rewards are large relative to the costs of pollen and seed production; in plants with flowers adapted to specialized, efficient movers of pollen; in species with wind-dispersed seeds; and in animal-dispersed species where seed dispersal is relatively independent of the number of seeds produced. These principles are supported by the distribution of breeding, pollination, and dispersal systems in various groups of seed plants.

The following summary table is based strongly on Table 1 from Givnish (1980), with some more current information from Eckenwalder (2009), reformatted for conciseness and updated to contemporary taxonomy:

Table 1. Gymnosperm breeding systems and dispersal syndromes. Number of species in each taxonomic group indicated in parentheses.

FamilyGeneraBreeding SystemDispersal Syndrome
Cycadaceae Cycas (88) Dioecious Fleshy seeds; rarely, water-dispersed
Stangeriaceae Bowenia, Stangeria (3) Dioecious Fleshy seeds
Zamiaceae All (168) Dioecious Fleshy seeds
Araucariaceae Araucaria, Section Araucaria (2) Mostly dioecious Winged cone scales
Araucariaceae Agathis, Araucaria (sections other than Araucaria), Wollemia (38) Monoecious Winged cone scales
Cupressaceae Juniperus (55) Dioecious Fleshy cone
Cupressaceae Austrocedrus, Diselma, Fitzroya (3) Dioecious Winged seed
Cupressaceae All except Juniperus and Austrocedrus, Diselma, Fitzroya (84) Monoecious Winged seed
Pinaceae Abies, Cathaya, Cedrus, Larix, Keteleeria, Nothotsuga, Picea, Pseudolarix, Pseudotsuga, Tsuga, and most Pinus (216) Monoecious Winged seeds
Pinaceae Pinus subsections Cembroides and Rzedowskiae, plus P. albicaulis, P. cembra, and P. pinea (15) Monoecious Seeds with a very small or vestigial wing, animal-dispersed
Podocarpaceae Acmopyle, Afrocarpus, Dacrycarpus, Falcatifolium, Nageia, Pherosphaera, Podocarpus, Prumnopitys, Retrophyllum (146) Dioecious Fleshy epimatium
Podocarpaceae Dacrydium, Halocarpus, Lagarostrobos, Lepidothamnus, Manoao, Microcachrys, Parasitaxus, Phyllocladus, Saxegothaea (37) Monoecious Fleshy epimatium
Sciadopityaceae Sciadopitys (1) Monoecious Winged seed
Taxaceae All (28) Dioecious Fleshy arils or seeds
Ginkgoaceae Ginkgo (1) Dioecious Fleshy seeds
Ephedraceae Ephedra (61) Mostly dioecious Fleshy cone bracts, wind-dispersed in 8 spp, otherwise animal-dispersed
Welwitschiaceae Welwitschia (1) Dioecious Conspicuous strobili with winged seeds
Gnetaceae Gnetum (32) Dioecious Fleshy seeds

Table 1 does not address pollination, presumably because Givnish (1980) believed all gymnosperms to be wind-pollinated; however, we now know that some cycads are also pollinated by insects, particularly beetles. All such species are dioecious, bearing seeds covered in a fleshy integument. Table 1 indicates that there are 588 dioecious and 391 monoecious species of gymnosperms; this count assumes all Ephedra are dioecious. Nearly all of the dioecious species bear their seeds within a fleshy receptacle that is suitable for consumption by animals, indicating that animals are the primary dispersal agent in these species; only 2 species, Diselma and Welwitschia, are dioecious but bear winged seeds, which are readily dispersed by wind. Diselma is native to the Tasmanian alpine country, and Welwitschia is native to the Namibian desert; both are places where wind is a singularly potent dispersal mechanism due to frequent high wind velocities and large expanses of open country. Conversely, nearly all monoecious species have winged seeds that are readily dispersed by the wind; the exceptions are a group of 37 species in the Podocarpaceae; and 15 species of Pinus in which the seed wing has been reduced to the point where it serves little functional purpose, and the plants rely upon animals for seed dispersal.

The questions remain, why are some gymnosperms monoecious and others dioecious; why are some dispersed by animals and others by the wind; and why does monoecy tend to accompany wind dispersal, while dioecy accompanies animal dispersal? A variety of hypotheses can be advanced, but I can find no evidence of experimental confirmation. Here are some points in the discussion:

Citations

Givnish, Thomas J. 1980. Ecological contraints on the evolution of breeding systems in seed plants: dioecy and dispersal in gymnosperms. Evolution 34(5):959-972.

Last Modified 2017-11-05