Thread by @FondOfBeetles

The Oxford English Dictionary cites several definitions of “sex”. As a noun, it refers to the categories into which humans and other sexually-reproducing species are divided on the basis of their reproductive function, and also to the act of sexual intercourse. 1/

As a verb, “to sex” is to determine the sex of an individual, although the OED also acknowledges “to sex up”, meaning either “attempt to arouse” or “make something boring look better”. 2/

In other words, in order to produce new organisms, a sexually-reproductive organism of a specific sex may sex themselves up, must correctly sex others of their kind, sex an individual up, and then have sex with that individual. 3/

Sexual reproduction – the generation of a new organism by combination of genetic material from two different parent organisms - is a 1.2bn years old process, and the vast majority of species on this planet reproduce sexually. 4/

It is an amazingly successful evolutionary strategy – the survival benefits of continued genetic mixing within a population of organisms far outweigh the physical burden of building and maintaining reproductive structures and systems. 5/

A basic recap of sexual reproduction. Adult cells have a “full set” of DNA (packed as chromosomes). For mating purposes, each parent makes cells that contain only a “half set” of DNA – we call these “haploid” cells/nuclei (“ha” = half, “ploid” = no. of chromosomes). 6/

When two adults mate, these two haploid cells fuse their haploid nuclei together and make a new individual with a reconstituted “full set” of DNA (which is different to the full set contained individually in either parent). 7/

In its most primitive form, sexual reproduction is the fusion of two haploid nuclei within cells/structures that have the same morphology in either parent – we call this “isogamy” (“same gamete”). Unsurprisingly, this tends to occur in lower organisms. 8/

Yeast are pretty lazy when it comes to sex. The "adults" make their haploid cells and just, well, dump them into the world. If one haploid cell happens to get near another, and they are compatible, they fuse and, yay, new yeast. Hardly worth a shared cigarette, to be honest. 9/

However, it is not always the case that isogametic sex is simply squishing together two haploid cells into a new cell. Some isogametic species have fascinating reproductive cycles with a marvellous array of structures and strategies. 10/

For example, many fungi form a living bridge between parents, with the nuclei from each transported into the suspended structure that is then pinched away from each parent and set free – fly, my beauties - as a spore. 11/

But, from living bridges (fungi) to parents that both produce little swimmers (algae), there is one thing isogametic species do not have, and that is “a sex”. 12/

Sexual reproduction involving two different gamete types is called “anisogamy” (“not same gamete”). The words “male” and “female” define the two different gametes involved in anisogametic sexual reproduction. 13/

Male gametes are small. Consequently, they cost little energy to produce, tend to be numerous and can be aimed in great number at their intended targets (whether that’s carried on bee legs or something rather more, ahem, direct). 14/

Female gametes are larger (in humans, 100,000 x larger). They take more energy than male gametes to produce, and are therefore biologically precious. Consequently, they tend to be made in fewer numbers and immobilised/hidden/protected in the individual producing them. 15/

The existence of two different gamete types has required the development of specialised reproductive structures required to produce and then either spaff or nurture such gametes. The definition of “male” and “female” encompasses these. 16/

In most anisogametic species, male and female reproductive structures are separated across two types of individual, who must then develop methods to recombine the gametes they produce. The definition of “male” and “female” encompasses these. 17/

The differential investment in gamete types has led to males and females within a species often looking remarkably different to each other. Typically, males are competitive and must win the favour of females to access her precious gametes. 18/

Thus, we have not only peacock tails and baboon arses but also antelopes that are too busy fighting each other to notice (care?) that one is about to get eaten by a lion. 19/

And thus, what we observe and recognise as "male" and "female" in the living world, from the way an organism looks to the way an organism behaves, is built from one single biological reality - gametes that can be distinguished by size. 20/

It is the lack of size-based differentiation of gamete types that preclude fungi from having a sex. In isogamous species, we refer to mating types, often using letters or numbers. Remember this the next time someone mentions 30k fungi sexes. 21/

It is the size-based differentiation of gamete types that allows us, upon discovery of a sexually-reproducing anisogametic creature, to name “male” and “female”. Remember this the next time someone mentions seahorses. 22/

As scientific definitions, “male” (small gamete) and “female” (large gamete) are no different to, say, “gold” (atomic no. 49). They are words used to describe something about the world, something that exists even in the absence of those words. 23/

I'll finish by noting, with some irony, that despite what you males been told all your lives, size really DOES matter. End/

PS. I know there are going to be biologists shouting ‘Not All Algae’ 😂 I know I know.