What on earth could that mean?
Geographic Parthenogenesis - a complicated sounding term referring to how an organism's sex life is affected by where it lives. Or what we can guess about an organism's sex life depending on where it resides. By sex life I am referring to whether an organism is sexual or asexual.
The distribution of asexual organisms (parthenogens) and sexual organisms in a population of creatures form consistent, distinct patterns. In comparison with their sexual relatives, asexuals tend to occur:
- further to the north (in the northern hemisphere)
- at higher altitudes
- in the outskirts of populations
- in environments with less resources
- on islands or island-like environments (as opposed to the mainland)
These patterns are known as Geographic Parthenogenesis.
An example in animals is the asexual species of grasshopper Saga pedo. It is the only asexual species in its genus and is found farther north than any of its related species. Many other examples are listed in biological literature - see references at the bottom for further information.
So why does it happen then?
There have been many theories for these patterns since 1928 when Vandel first noted that asexuals and sexuals often occupy different areas of an environment. The main views relate to differences in ploidy, differences in ability to colonize, "environmental uncertainty", and Source and Sink Populations. I shall now outline these views.
Tough Asexual Polyploids
Ploidy refers to the number of sets of chromosomes that are contained in an organism's genotype. Haploidy is when only one set is present - most commonly in asexuals. Diploidy is when two sets of chromosomes are present - most commonly in sexuals. polyploidy, which refers to genotypes with more than two sets of chromosomes, increases the total amount of DNA in the genome, providing material for the evolution of new attributes and abilities. Polyploids are most usually asexual because if a mutant asexual occurs with extra copies of chromosomes it does not have to find another organism in a similar, rare, condition with which it can successfully reproduce. Organisms need to be of the same ploidy to produce offspring together.
Bierzychudek argues that it is difficult to separate patterns relating to ploidy and those relating to breeding systems (i.e. whether an organism is asexual or sexual). She notes several experiments in which plants with higher ploidy have had better competitive ability - making them more able to deal with harsher conditions.
However, some of the patterns mentioned above do not imply a harsher environment as such. For example, the outskirts of populations are not necessarily harsher but instead consist of an area where the population is distributed more sparsely.
Having trouble finding a mate...
Tomlinson wrote that asexuals can be favoured above sexuals in scattered or reduced populations where the likelihood of finding a mate is low - asexuals reproduce on their own so they require no mate. A similar idea is that asexuals inhabit islands (as opposed to the mainland) because it is easier for them to start new populations, needing only one individual. Once the population is started it increases quicker than a sexual population because only females need to be produced (the two-fold cost of sex).
However, Bierzychudek pointed out that the colonisation idea cannot fully account for Geographic Parthenogenesis, because it does not explain why the asexuals do not colonise the areas where the sexuals are or why the sexuals do not eventually spread further into the areas currently inhabited by asexuals.
Actually it's the sexuals that live in difficult environments!
Glesener & Tilman believe that the distribution patterns of sexuals and asexuals can be explained by "environmental uncertainty". They argue that sexuals can adapt better to situations where it is harder to predict what kind of environment offspring will encounter in their life. These often occur when an environment is particularly inhabitable so more types of organism are local to that area. This leads to greater competition between species and uncertainty over which of the wide range of organisms an offspring could meet. The asexuals can only survive in less hospitable areas where competition is lower.
Once again Bierzychudek is not convinced. She investigated this point of view by looking for coexistences of asexuals and sexuals, commenting that with this viewpoint we would expect sexuals and asexuals not to coexist, but for the sexuals to dominate. She found that they commonly coexist and suggests that this hypothesis is too simplistic.
Latest Research: Source and Sink Populations
Peck et al. explain the phenomenon in terms of the movement of individuals from areas where they are well adapted to those where they are less well adapted. There are more individuals produced in areas where the population as a whole is better adapted - these are called source populations. Areas that organisms from the source population migrates to are called sink populations. When an organism migrates, it generally moves to an area where it is less well adapted, making it a less viable mate - its offspring will have less chance of surviving than organisms local to the area. Since sink populations, by definition, contain many migrants, asexuals in these areas will be more likely to leave offspring which survive. Eventually, asexuals come to dominate these areas, which correspond to the patterns observed in geographic parthenogenesis.
The model used by Peck et al. required sexuals to be inherently better than asexuals, several reasons have been published for this: reproducing sexually is thought to gives greater resistance to parasites, helps the accumulation of positive mutations and stops the build up of deleterious mutations that has been associated with asexual reproduction.
One of the advantages of this model is that it can deal with all patterns of Geographic Parthenogenesis. For example, the greater ease of colonisation for asexuals hypothesis (having trouble finding a mate...) does not explain why asexuals occur further north. The model based on Source and Sink Populations explains this because further south the environment is more hospitable so an organism can devote more resources to producing offspring, thus creating a source population. Further north where there are less resources, fewer offspring are produced. The north then becomes a sink population which the southern organisms migrate into.
- Bierzychudek, P.; "Patterns in plant parthenogenesis", Experientia 41, 1255-1265 (1985).
- Glesener, R. R. et al.; "Sexuality and the components of environmental uncertainty: Clues from geographic parthenogenesis in terrestrial animals", Am. Nat. 112, 659-673, (1978).
- Lynch, M.; "Destabilizing hybridization, general-purpose genotypes and geographic parthenogenesis." Q. Rev. Biol. 59, 257-290, (1984).
- Maynard Smith, J.; "The Evolution of Sex", Cambridge University Press (1978).
- Peck, J. R.; "Limited dispersal, deleterious mutations and the evolution of sex", Genetics 142, 1053-1060, (1996).
- Peck, J. R. et al.; "Explaining the geographic distributions of sexual and asexual populations", Nat. 391, 889-892, (1997).
- Suomalainen, E.; "Parthenogenesis in animals", Adv. Genet. 3, 193-253, (1950).
- Tomlinson, J.; "The advantages of hermaphroditism and parthenogenesis", J. Theoret. Biol. 11, 54-58, (1966).
- Vandel, A.; "La parthenogenese geographique: contribution L'etude biologique et cytologique de la parthenogenese naturelle", Bull. Biol. Belg. 62, 164-281, (1928).