Sex in the Tree of Life

The demarcations of human sexuality have become a major issue in the culture wars, but for plants, sexual diversity is the norm. There are plants with “perfect” flowers that are completely hermaphroditic, with fully functional pollen and eggs produced in the same flower. There are monoecious plants, which produce both male and female flowers. There are dioecous species, with individuals that only produce male or female flowers. And then things start to get complicated. Gynodioecy is the phenomenon in some plant species in which individuals are either female or hermaphroditic. A student in Jennifer Blake-Mahmud’s Sex in the Tree of Life class converted the short, one-paragraph article on gynodioecy into a substantial, informative article.

Some of this diversity is on display in the genus Silene, a widespread genus of small wildflowers. In addition to monoecious, dioecious and gynodioecious species, Silene includes trioecious and andromonoecious species. Some even display more than one type of sex determination. If you want to know more about all of this, check out the sex determination in Silene article that a student in the class created.

When people think about sexually transmitted infections, they rarely think about plant disease, but that’s precisely how Microbotryum violaceum infection of Silene latifolia is usually classified. Silene latifolia is a small flowering plant that is native to Europe, western Asia and North Africa. It was introduced into North America and has become widespread. Microbotryum violaceum is a smut fungus, a parasitic fungus which takes over the anthers of infected plants and uses them for spore production. Pollinators visit the flowers and transmit the fungal spores instead of pollen (thus making the infection sexually transmitted). This article is also the handiwork of a student in the class.

And then there are figs. Figs have a complex reproductive cycle in which fig wasps lay their eggs in fig flowers. The wasp larvae parasitize the flowers, and female wasps emerge covered with pollen and go off to find another fig to pollinate, lay their eggs in, and die. Almost every fig species (and there are about 800 of them) is pollinated by a single species of fig wasp. Both fig and fig wasp are completely dependent on one another in order to reproduce, and unsurprisingly, pairs of fig and fig wasp species have coevolved. If you want to learn more about this, you can check out the reproductive coevolution in Ficus, which was also created by a student in the class.

While natural selection and sexual selection are thought of as the main drivers of evolutionary change, social selection has been proposed as an alternative to sexual selection. While sexual selection applies to mate choice, and puts the choice in the hands of only one gender, social selection is transactional – one individual offers something in exchange for the opportunity to reproduce. Thanks to the work of a student in this class, you can now learn more about this model of evolutionary change on Wikipedia.

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Image: 20140427Silene latifolia2.jpg, by AnRo0002, CC0 1.0, via Wikimedia Commons.


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