Getting plant sex wrong

From 28 Oct 2009.

Watching “Botany of Desire” on PBS. I’ve generally been a bit ambivalent about Michael Pollan, but about 36 minutes in he veers into “just plain wrong” territory. “Before that [before the evolution of angiosperms] you had this greener, sleepier world where things reproduce usually by cloning, by spores that were genetically identical to their parents”. Regarding cloning–yes, there were a fair number of clonal plants before angiosperms evolved, but there are also plenty of clonal angiosperms. Regarding spores–he is right that sexual recombination isn’t involved in producing spores but: 1) this does not mean the spores are genetically identical to the parent–they aren’t; 2) angiosperm reproduction involves the production of spores as well, so this is not a difference between angiosperms & non-angiosperms.

Brief recap of plant life cycles: the dominant portion of the vascular plant life cycle is the sporophyte (on the other hand, gametophytes are dominant in non-vascular plants: mosses, liverworts, hornworts), which is diploid (has two sets of chromosomes, just like all stages of the human life cycle except sperm & eggs). The diploid sporophyte produces spores by meiosis. Meiosis halves the chromosome number, so the spores are haploid. Whereas the sporophyte has two copies of each gene (excluding the rare cases in which plants have sex chromosomes), each spore has one copy of each gene. It’s the same as the relationship between, for instance, a human male and one of his sperm cells (except, for the sake of nit-picking, that humans have sex chromosomes and most plants don’t), and is not genetic identity. However, whereas human sperm & egg cells do nothing more than unite to form a zygote, plant spores undergo cell divisions to produce gametophytes. Gametophytes are a multicellular haploid stage in a vascular plant’s life cycle, and they produce gametes (sperm and eggs) through mitosis. Gametophytes exist in all plants, but are quite small and dependent on the sporophytes in angiosperms. Pollen grains are male gametophytes. Inside each ovule lives a female gametophyte. When the sperm and eggs produced by gametophytes join in fertilization, we are back at the diploid sporophyte level. Very short version: diploid sporophyte produces spores; spores grow into haploid gametophyes; gametophytes produce sperm & eggs; fertilization takes us back to a diploid sporophyte. Although (with minor exceptions; e.g., the genus Vittaria, some species of which exist solely as gametophytes) all plants produce spores, in no plants is the production of spores itself sexual. The production of spores is part of the sexual process, however; without the gametophytes they give rise to, you can’t get gametes. Lest you think Pollan just misplaced a word or two, he continues: “And then you have this incredible explosion of diversity that happens with this new strategy. It was an incredibly successful strategy. It allowed you [by which he means angiosperms] to move your genes around, it allowed you to evolve much quicker because sex creates variation.” Nope, he didn’t just misplace a word or two, he’s really saying that the key innovation of angiosperms relative to earlier plants was sex.

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