What are the two sexes of a plant moss?

That’s cool when requests coming here are direct questions. Eventually turning into posts of some sort…

Such a recent question was “what are the two sexes of a plant moss?”. Well, the two basic sex functions are male and female, as long as gametes can be differentiated (big gametes are defined as female, small ones as male –both or none may be motile in plant species, so being able to move is not a feature that we can ascribe to a male gamete as we do in animals). That said, we know flowering plant species can express a wide diversity of co-expression of these sexual functions, resulting into sex phenotypes that vary from perfect hermaphroditism to a complete separation of sexes (that’s for the plant sex post).

The thing is that mosses are somewhat special. They spend most of their life cycle at a haploid stage (having only one set of chromosomes), while the diploid stage everyone regards as normal is a transient part of their life: the erect organ bearing and dispersing spores, whose morphological diversity is illustrated on your right (picture from Wikipedia, it’s a drawing from the very famous biologist Ernst Haeckel).

So when we want to ascribe sex to moss species, we are looking at something somewhat different than what we are used to (the phenomena is called alternation of generations). The diploid stage, in which meiosis takes place (well, that’s an important part of sexuality, though only biologists seem to care about this and others dismiss it as an uninteresting step of sex), is not sexable. On the other hand, the haploid stage is. But at this point, cosexuality like we observe in flowering plants never was really reached in mosses. That is, sex organs are different and either male or female and never co-locate immediately next to each other in an verticillate organized fashion like in many flowers. The number of amenable possibilities is thus reduced, but there’s still some diversity: either sexes will separate, with male and female individuals, and in this case the species is said dioicous (in parallel to dioecious in flowering plants). Or sex organs may be produced on the very same individual but different places apart, and in this case the species is said monoicous (in parallel to monoecious in flowering plants). Where sex functions can be expressed on the very same plant (monoicous species), there is some more diversity that furthers the jargon: when sexual structures are clearly separated in different clusters, that’s autoicous (or even cladautoicous when sex clusters are found on different ramets from the same individual). Where male and female sex organs are temptatively produced very next to each other in an inflorescence-like structure (the morphologically closest thing we have to flowers), that’s synoicous (or also androgynous). Finally, some species are not completely decided yet as to whether they should be monoicous or dioicous and are found to vary in this regard. These species are said heteroicous.

So what are the two sexes of moss plants? Well, hum, simply male and female, because moss species never achieved hermaphroditism like flowering plants did. Even when they try hard, monoicous is almost the best they reach. Or say, androgynous. I would really like to see if reproductive insurance is the factor promoting the evolution of androgynous inflorescences in mosses, but I don’t know of any relevant paper yet. Because of instances of clonality, which can locally be fairly important, some moss species are naturally selfing at high rates without the need to evolve specialized sex structures. Of course, they need clonality and genetical loneliness in the environment to do so, but it doesn’t seem to be that uncommon.

Other moss sex trivia of interest:

• Apparently, about half moss species are monoicous and half are dioicous.
• Monoicous species have higher heterozygous defiencies than dioicous species (i.e. they indeed self) [1]
• Dioicous species can express inbreeding depression, while monoicous species may not, indicating that recurrent selfing allows to purge deleterious mutations while outcrossing keep them warm in the gene pool [2]

To go further, one might wish to know how to sex the moss plants. Following this other scheme (from Wikipedia too), it appears you can morphologically differentiate male from female plants by sight in dioicous species like Polytrichum (well, I guess this is the Genus featured in the scheme). So what about the moss pix below the scheme?

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Not easy, eh? So here is a better trick, if you’re really patient and want to be sure: wait for the moss to produce seta. Plants with seta necessarily are female. The others are either male, or maybe female that didn’t reproduce this year. That’s easier, isn’t it? :-)

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[1]- S.M. Eppley, P.J. Taylor and L.K. Jesson (2007). Self-fertilization in mosses: a comparison of heterozygote deficiency between species with combined versus separate sexes. Heredity 98(1):38-44.

[2]- P. J. Taylor, S. M. Eppley, and L. K. Jesson (2007). Sporophytic inbreeding depression  in mosses occurs in a species with separate sexes but not in a species with combined sexes. American Journal of Botany 94(11): 1853–1859.