Dioecious

One of the best pleasures lately has been reading Tudge's 'The Secret Life of Trees' in combination with his 'Variety of Life'. The latter is largely a reference work, but the former is an in-depth romp through one plant-form- a tree. Trees are not a clade (or phylogenetically cohesive grouping), but tree-ish plant-forms have arisen independently in many plant orders. Tree (a descriptive term) connotes a lignified and large plant.

When you've been beastie-focussed, learning more about plant reproduction starts to blow your mind. Those plants have all sorts of ways of making it happen, and one of the oddest ways is to do it as we humans do- with single sexes in single organisms. In 95% of angiosperms (flowering plants) there is some hermaphroditic overlay, with the most common pattern being male and female organs in the same flower or on the same plant (a monoecious reproductive style). The dioecious species like we and most animals are the outliers.

Those weird dioecious species are found among; Holly, Gingko, Juniper, Cannabis, Date Palm, Persimmon, Sago, Kiwi Fruit, Yew, Poplar, Willow, Spinach and Asparagus.

In part, dioeciousness seems a functional extension of self-incompatibility, a trick played by many plants to prevent self-fertilisation. Sexual self-fertilisation is not the same as vegetative cloning. The gametes have undergone meiosis and gene transfer between chromatids during their formation, and thus are not genetically identical to their mother. However, sexual inbreeding (self-fertilisation) is nowhere near as successful in promoting genetic diversity and adaptive potential as sexual outbreeding. Dioeciousness promotes this de facto.

Why did dioecious habits evolve in some species? To protect sexual fertilisation one suspects, and promote the mixing up of genes. In addition, dioecious habits could be promoted by symbiotic co-evolution between the plants and their broadcasters/ germinators. Many dioecious plants are dependent upon birds eating their fruits and enclosed seeds, preparing seeds through their digestive system and depositing these for germination at a distance, away from direct competition with their parents but a potentially successful descendant.

Upcoming Exclusives:

* The triploid nuclei in pollen of angiosperms- menage a trois!
* Plant embryogenesis- all about sex!
* The pollen tube, the endosperm and the importance of mechanics- Viagra for plants!
* Seed plant reproductive organs- angiosperms and gymnosperms embryologically unrelated and probably legal!