Why genetics is great for studying bryophytes

Bryophytes are wonderful plants and I would like to think that I do not need to do much canvassing on their behalf. Besides being fascinating in their own right, bryophytes have some properties that make them the ideal study objects for genetic research. In this post, I am going to tell you why.

Small size
First of all, bryophytes tend to be small. Their small size might may turn away some workers. After all, one has to bend down a lot, any serious study of bryophytes requires the use a hand lens if not microscope, and it is sometimes said that bryophytes do not have many characters that differ between species. But small size also means that many individuals can grow in the same spot. Imagine the space taken up by an oak tree, a gorse bush, or even just a tussock of grass and think how many individual Bryum, or Polytrichum plants could thrive in an area of the same size. Larger populations can maintain higher levels of genetic diversity, and this is seen in many species of bryophytes. Another advantage of small size it that bryophytes can be cultured in the laboratory, where they may be kept in permanent culture. They also do not need much by way of nutrients and can be cultured for long times in hermetically sealed containers.

Dispersal (gene flow)
Bryophytes cover a wide gradient of dispersal capability, perhaps wider than any other group of land plants. Many bryophytes disperse by means of tiny, long-lived, airborne spores, which may be carried across oceans and continents. In these species, gene flow may “glue together” distant populations preventing them from diverging genetically. Other species have heavier spores, which do not travel quite so far. Yet other bryophytes are sterile and disperse only asexually (also called vegetatively) and presumably even less far. Finally, there are some spore-producing species, whose capsules never open. Examples are found in Aphanorrhegma (Physcomitrella) and Riccia, which are called cleistocarpous. These different dispersal capabilities are likely to affect the extent of genetic divergence of geographically distant populations. The less gene flow, the more divergence now would expect.

Mating system
A large proportion of bryophyte species has separate sexes. This is far more common in bryophytes than in flowering plants. The norm in flowering plants (angiosperms) is the possession of flowers containing both male and female parts. While many flowering plants have self-incompatibility mechanisms (genetically very interesting) ensuring outcrossing to happen, there are plenty flowering plants which can self-fertilise. This ability to self-fertilise is often seen as an advantage – one individual blown to a remote island may give rise to a whole new population, while a single male or female individual could not reproduce. Self-fertilisation tends to reduce a population’s genetic diversity. It may be beneficial in the short term to generate a large population, but lacking genetic diversity (the material for natural selection to work on), such a population may struggle to adapt when environmental conditions change. The possession of separate sexes makes it impossible to self-fertilise, representing another bryophyte mechanism of maintaining high levels of genetic diversity. Because many bryophytes are much better dispersers than flowering plants, the situation where one individual arrives in a new territory without a suitable mate, probably matters less for them. That said, there are some bryophyte species where only one sex is ever found in some geographic region, where they spread vegetatively.

Distribution of bryophyte diversity
As a general pattern, plant biodiversity is higher the closer one comes to the equator. This pattern of higher diversity at lower latitudes is not so clear in bryophytes (and in particular in mosses) where diversity can regionally increase with latitude, as seen in Europe. While being a relatively small archipelago far from the equator, the British Isles (where I happen to be working) are a great place to study bryophytes. It is not the most bryo-diverse place, but it has got a very high species density per area. Additionally, the British bryophyte flora is extraordinarily well-documented. This includes a wealth of historical records and herbarium samples.

Global change
The global distributions and population sizes of many plant species are changing, and this is seen in bryophytes, too. The change in air quality since the reduction of sulphurous emission seems to allow several bryophyte species to reclaim urban areas from where they had disappeared for more than a hundred years. Also, climate change alters the suitability of geographic areas for some bryophyte species, causing their ranges to change. Once inhabited areas may be abandoned, and new areas becoming suitable may be colonised. This raises several questions. Are newly invaded areas poorer genetically? Are species and genetic diversity lost when regions long inhabited become unsuitable? In the case of the British Isles, surrounded by sea, and with relatively low mountains, species may disappear, because they may have nowhere to go. What is the rate of arrival of new species? There also are invasive bryophytes. These are species that were introduced by human action (including inadvertent introductions) and which spread at the cost of the local vegetation or cause an economic cost. Interesting old questions, which are still unanswered in flowering plans are: Is invasiveness due to some genetic predisposition? Do invasive plants adapt to their new environments rapidly? If so, is that using genetic diversity already present, or through novel genetic variants? Does hybridisation tend to be involved in this process? Are invading lineages different (genetically less diverse) compared to the same species in their original ranges? All these questions are relevant to bryophytes too, and bryophyte are well-suited to study and answer them.

Small genomes
In the age of genomics where whole genomes are sequenced and trait differences can often be associated with the causative genetic variants, it is beneficial to work on species with small genome sizes. We do not know as well about bryophyte genome sizes as we do about those of flowering plants because fewer genome sizes estimates exist for bryophytes. So far, bryophyte genome sizes seem to be smaller on average, which I find good news because it makes sequencing bryophyte genomes more affordable. This does not mean that bryophytes lack genomic complexity or genomic phenomena commonly seen in other plants. Bryophytes have transposable elements (selfish DNAs) and polyploidy (whole genome duplication) is seen in several lineages of bryophytes. Hybridisation, too, is reported increasingly.

Dominant haploid generation
My final point also has to do with bryophyte genomes. It is haploidy. The dominant stage in the life cycle of bryophytes is usually the gametophyte. This is the plant that makes the male or female sex cells (spermatocytes or oocytes), very much like human sperm or egg cells and the egg cells, pollen and pollen tubes of seed plants. The difference in bryophytes it that this haploid generation is not ephemeral. Different to pollen, eggs, and sperm, the haploid gametophyte is what we are used to call the moss or liverwort (or hornwort) plant. It represents the main life stage of most species and it is the life stage from which the DNA for genetic analyses is usually extracted. This simplifies genetic inference a lot. When sequencing other plants and animals, where the main life stage is diploid, the DNA extracted from one individual is made up from two genomes, making it very challenging to separate the DNA variants found into two distinct genomes. In fact, most genome reference sequences in public repositories like GenBank are chimeric and contain bits of both genomes (or even multiple individuals). This does not matter much if the purpose of the deposited genome sequence is to serve as a reference (like a blueprint), but it matters a lot when analysing the ancestry of genetic variants in a population, one of the most exciting and rewarding aspects of genetic research. (There may be some personal bias in this last statement.)

Summary
Bryophytes have features that make them interesting objects for genetic studies. These include a wide gradient of dispersal capability, high genetic diversity, wide distributions, various mating systems, and ongoing range changes. Bryophytes are also particularly suited to be studied with genetic (and genomic) methods, because they tend to be small, with small genomes, and they are haploid. These advantages make bryophytes excellent candidates for the study of questions relevant to all land plants.

Digging deeper

  • If you are interested in plant genome sizes, check out the plant C-value database (maintained at Kew Gardens). There is a related (open access) publication by Pellicer & Leitch (2020). Their figure 2 is worth having a look at, showing genome size distributions of different plant groups (note the logarithmic scale).
  • The recently described species Ceratodon amazonum is an example for a moss with separate sexes, where only females are ever found. The species also hybridises with the well-known C. purpureus:  Nieto-Lugile et al. (2018). At the time of publication of this article, C. amazonum had not been described and it is referred to as “the new species”. If you do not have access to the full text, you can follow the link to the “Author manuscript”.

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