I was shocked (though, I suppose, not surprised) when the Supreme Court overturned Roe v Wade. Over the past few weeks I’ve been reading opinion pieces for people in favor of the decision, trying to understand why people view this as a moral victory. While I don’t believe in objective morality, and I definitely don’t think we should defend or justify cultural decisions by looking to plant and animal examples (we’ll get to animals next week), abortion and related reproductive strategies that terminate pregnancies early are incredibly common and widespread across various taxonomic groups. In the context of natural history, abortion is a reproductive and life history strategy, not a moral choice. In the end, it doesn’t matter whether this is a moral or an ecological decision, so long as it is up to the mother to decide.

Terminating seeds for dispersal

A few years ago, I was with a group of kids collecting large amounts of sugar maple samaras (those paired winged fruits) to throw off a bridge. One of the students noticed that in each pair of samaras, one of the samaras was missing a seed. The pattern held for virtually all pairs of samaras indicating that this was a strategy and not a defect. It turns out that while both seeds begin to develop, only one seed reaches maturity while the other is quickly terminated by the parent plant (yes, botanists describe this as behavior, connoting an active process).

Each pair of maple fruits is shed by the tree as a single unit. The offset weight distribution of the lopsided samaras helps throw off their trajectory, sending the seeds careening farther away from the parent plant than if it was evenly balanced. Here a full half of the fertilized embryos on a sugar maple are sacrificed to carry the other seed farther from the parent plant.

Female (left) and male (right) flowers on a sugar maple (Centennial Woods)
Twin sugar maple samaras freshly fallen (Underwood Property, South Burlington)

In the wind-dispersed tropical tree, Indian rosewood (Dalbergia sissoo), seeds may compete for energy resources in fruits with numerous fertilized embryos. The first embryos within the fruit to be fertilized (these are near the tip of the fruit), produce a hormone that aborts embryos near the base of the fruit. In rosewood fruits with fewer developing seeds, competition is lower and the hormone is not released. Fruits with fewer seeds also disperse farther from the parent plant (source). Again we see the process of aborting seeds as an active adaptation for seed dispersal.

Seed abortion and environmental stress

Inside the female part of a plant (the pistil) lies one or more unfertilized embryos. In order to develop into a seed, the embryo must fuse with a sperm cell (released by a pollen grain). The stigma (the sticky tip of the pistil) is a sort of gatekeeper, rejecting pollen from other species and, in species that require cross-pollination, the stigma even rejects pollen from the same individual plant. If the pistil gives the pollen the green light (a biochemical signal), the pollen grain releases sperm with the hope that they might fertilize an embryo.

While a female flamingo might be able to judge the health of a male by looking at their plumage (source), the stigma can’t tell much more than the compatibility of the pollen grain. The plant risks wasting energy developing seeds doomed to fail (due to genetic defects). This is particularly risky for female plants who then bear the energetic burden of “raising” the young.

Aborting seeds can also function to increase the overall survivability of a plant’s seed crop by sacrificing some seeds as a sort of false advertisement to potential predators. Say a plant aborts 50% of its seeds. This would mean that 50% of the seeds encountered by foraging seed predators (like finches or mice) will be a waste of predator’s time and energy, which ultimately deters future predation on the seeds. These aborted seeds may be genetically inferior (e.g. self-fertilized seeds: source).

A procession of bright red aphids sucking sap from a sunflower (Backyard, Burlington)

For some species, when the fruit is parasitized, the whole fruit may be terminated and shed early rather than just the seeds. Commercial sunflower (Helianthus annuus) is occasionally parasitized by broomrape plants (Orobanche spp.). Parasitized sunflowers have smaller flower heads, lower oil production, and aborted seed heads than healthy flower heads. The plant effectively shunts resources away from an infected part of the platn. The same thing occurs in barberry (Berberis vulgaris). Shrubs parasitized by the specialist fruit fly, Rhagoletis meigenii, can lose up to 100% of their young. However, if a barberry aborts most of its seeds when it is first parasitized, the plant can successfully produced a crop (source)

Dangling racemes of common barberry with bright red berries (Essex Dam)

The consistent theme here is that reproduction is always a cost, and a cost that must be weighed by females against the other background essentials of growth, maintenance, defense, that also require energy, nutrients, and time. 

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