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Case Studies

Genetic diversity

It is often said that one of the benefits of sexual reproduction is the generation of genetic diversity, through the mixing of genes from two parents. Since vegetative reproduction does not involve any such mixing how could vegetative propagules show any genetic diversity?

Growth in a bryophyte gametophyte is controlled by what are called apical cells and there's more about this in the APICAL CELL CASE STUDY. The rest of this paragraph will be devoted to a very brief summary of the concept. At the apex of a moss stem, at any given time, there is a single cell which is the source of all future growth of that stem. If a moss stem branches to produce two stems, each of those will have its own apical cell. There is nothing specific to mosses or stem tips in such apical cell influence. Any lobe of a thallose bryophyte will have an apical cell. A leaf growing from a moss or liverwort stem has its own apical cell. If a chromosome in an apical cell undergoes a mutation, that mutation will be passed on to the succeeding cells, making them all genetically different to those produced before the apical cell mutation. Propagules from the post-mutation part of the plant would give rise to gametophytes genetically different to those produced by any propagules released from the pre-mutation part of the original gametophyte.

Do spores imply genetic diversity? There are many dioicous bryophyte species, but also many monoicous ones. In a monoicous species both the eggs and sperm are produced on the same plant, whereas in a dioicous species there are separate male and female plants. Suppose a monoicous gametophyte has not undergone any mutation and is now producing sperm and eggs. Every cell in the gametophyte is genetically identical. The gametophyte is haploid, which means that all eggs and sperm are also genetically identical. If there are no other gametophytes close enough for their sperm to reach the gametophyte we're discussing, sporophytes could still be produced, but only through self-fertilization of this monoicous gametophyte. The resulting spores would be genetically identical to the parent gametophyte. In this scenario the spores are no different to vegetative propagules, albeit vegetative propagules that, if of small size, could be capable of much longer distance dispersal than the other vegetative propagules produced by the gametophyte.

What if there are some gametophytes close enough to fertilize the particular one we're interested in? That still need not imply genetic diversity in the resulting spores. What if all the nearby gametophytes arose from vegetative propagules dispersed from the gametophyte we're focussing on? They could all be genetically identical to their parent, in which case the results would be no different to the previous case of self-fertilization. Going back to the case where self-fertilization occurs, what if there has been some apical cell mutation? Depending on which apical cells underwent mutation, the egg or sperm produced on one stem or lobe could well be genetically different to those produced on another. In this case, self-fertilization could lead to genetically different spores.

The frequency of events such as mutation, vegetative reproduction, self-fertilization and sexual reproduction via two parents may vary from one species to another or, even within a species, from one colony to another, depending on local conditions. The intention of this discussion is simply to point out that it is wrong to automatically assume that spores always imply genetic variety and that vegetative reproduction never does.

Mutation of cells other than sperm or egg cells is termed somatic mutation, so apical cell mutation referred to earlier is an example of somatic mutation. A number of bryologists have argued that somatic mutation in bryophytes could generate as much genetic diversity as that generated by sexual reproduction. Other bryologists, while allowing an important role for somatic mutation, are not convinced of the truth of the claim made in the preceding sentence.