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Monday, February 5, 2018

Speculative Biology: 3-Way Reproduction

The other day I found myself thinking about what would be the fundamental biological characteristics of a species having a system which depended on three individuals, instead of the two or one we're used to, for each reproduction event. (This is a distinctly different concept from a species having multiple sexes or genders. See references at end for examples of these.)

To simplify the discussion, I'm going to start with a big assumption that the hypothetical organism only differs from what we see here in that it has a 3-way reproduction system. It is carbon based, it has DNA organized into chromosomes, etc. Breaching that huge assumption would introduce far more variables into the discussion, when I'm only interested in the basics of sexual reproduction for this discussion.

There are different ways for Earth biology to control the sex of individuals. Sex in some species is driven by genetic differences (mammals and birds). In some it is driven by the number of chromosomes (bees and wasps). In some it is driven by temperature differences (reptiles and amphibians). In others the sex changes with age or social situations (some fishes). In large groups, there are almost always exceptions to the general patterns.

Almost all of these cases involve some chromosomes being contributed to an offspring by both a female and a male. Haplodiploidy in bees/wasps is an interesting exception. (Males grow from unfertilized eggs, while females grow from fertilized eggs.)

For our 3-way reproduction discussion, lets start by assuming each of the three individuals contributes chromosomes equally to each offspring. (Later we'll examine a more complicated case.)

We can easily enough abstract the concept of a Punnett square into higher dimensions, though it does get difficult to simply convey the results in a 2D format. In the regular version, the possibilities for a single chromosome contributed from each parent are aligned along the top and side edge of the table. For a 3D version we'll do the same, but split the contributions from the third parent into three sub-tables (left to right) with the third parent contribution at the upper-left corner of each. (I've added some color highlights to help visualize the contributing parent for each chromosome as well as the sexes of the potential offspring.)

Punnett square (and "cube") for 2-way and 3-way crosses.

The first observation that stands out from this is the difference in predicted sex ratios of the offspring. In our 2-way system, the calculations implies a 1:1 ratio. In the 3-way system, the calculations implies a 1:2.5:1 ratio between the three sexes of offspring.

Next lets discuss something analogous to the haplodiploidy of bees/wasps, where males only contribute chromosomes to their daughters.

Punnett square (and "cube") for 2-way and 3-way crosses, with haplodiploidy.

This shows the same possibilities for sex ratios of offspring. Since we're using bees as a model here, it's a good time to introduce the idea that a creature doesn't have to produce offspring at the ratios suggested by such simple calculations. Bees produce very few males, and only when needed for fertilization of new queens. Similarly, a hypothetical 3-way reproducing species could easily adjust the sex ratio of its offspring to be different from what the above calculations suggest.

An abstraction from Fisher's Principle (http://the-biologist-is-in.blogspot.com/2015/12/evolutionary-battle-of-sexes.html) suggests most species would evolve towards a 1:1:1 ratio between the three sexes. Cases where this wasn't the case would be interesting.

I imagine one sex evolving into an approximation of female (with a large immobile gamete), while the other two sexes evolving into an approximation of male (with smaller, more mobile gametes). It gets much more difficult to make predictions beyond this point, though a couple examples inspired from fiction and biology come to mind.

Maybe the two male-equivalents would actively court each other and then seek out the a female-equivalent together as a pair. This seems to be the pattern described for the fictional Pierson's Puppeteers (though their sexual biology is rarely detailed in the author's stories about them).

Maybe the male-equivalents would independently seek out the female-equivalent. I imagine something similar to the Deep-sea Anglerfish, with smaller male-equivalent individuals fusing to a larger female-equivalent and waiting for the opportunity to contribute to offspring when all three sexes have joined the party.

In the grand scheme of things, I expect biological systems requiring three individuals for reproduction would be rare in the cosmos. At an early evolutionary stage, any organisms which only required two partners to reproduce would probably out-compete those requiring three partners simply because it would be easier to arrange appropriate matings. This isn't to say it wouldn't happen, since all sorts of strange things happen in biology.

The Red Queen hypothesis (http://the-biologist-is-in.blogspot.com/2014/04/oxalis-and-red-queen.html) suggests why larger species don't simply have one sex. Yet, we do see this from time to time.