The Importance of the Reproductive Techniques of Poecilia formosa

One of the most interesting things in the world of vertebrates is what happens when you have a hybrid of two species. When this happens you tend to get many different kinds of results, some lasting only one generation, some lasting two, and then there are the ones that just keep going. The success stories if you will. This is the case of what will be presented in this paper, in the specific case of Poecilia formosa. In this paper the reproduction and possible origins of the live bearing fish Poecilia formosa will be explored, as well as its importance to the scientific community in terms of it being: desirable for genetic research; desirable for clonal research; and a possible application of Muller's ratchet and methods in which asexual animals have adapted in order to disable the ratchet.


Basic Reproduction

The Amazon Molly, Poecilia formosa, reproduces through gynogenesis.

Gynogenesis: Clonal (asexual) reproduction that is stimulated by the sperm of another, closely related species, sometimes referred to as the sexual host. Another name for gynogenesis is sperm-dependent parthenogenesis.

This means that although Poecilia formosa must mate with a male, it does not incorporate the males additional genetic material into the already diploid (2n) egg cells that the mother is carrying except in extraordinary circumstances, resulting in identical clones of the mother being produced en mass. The presence of the sperm is only necessary to stimulate the already diploid eggs cells into embryonic development. This unusual characteristic has led to Poecilia formosa becoming an all female species up to this point, because there is no need for a male child to be produced when they are not needed to guarantee the survival of the species. The common name, Amazon Molly, acknowledges this trait as a reference to the Amazon warriors, a female run society in ancient history.

In nature, Poecilia formosa typicaly mates with a male of one of three or four different species, either P. latipinna¹, P. mexicana¹, P. latipunctata¹, and occasionally P. sphenops². There is also one other male that could possibly exist in Poecilia formosa's natural range that could induce parthenogenesis in P. formosa females, and that is triploid (3n) P. formosa males. These triploid males are very rare in nature and are not necessary in the reproduction of the P. formosa species, which is why the species is considered to be all female. The Amazon molly reaches sexual maturity anywhere from 1-6 months after birth and typically has a brood (batch of young) with somewhere between 60-100 fry (young) being delivered every 30-40 days. This lends itself towards a large potential for population growth as long as the host is present. The wide variability in maturity dates and brood sizes are a result of genetic heritage, varying temperatures, and food availability. They will become sexually mature faster and produce larger broods in warm (approximately 80¼F) water that provides an overabundance of food.


Possible Origins

In almost every case P. formosa is assumed to be a hybrid of two of its most closely related hosts. The main problem with this theory is that it has been impossible to prove thus far as no known hybrid has ever had the all female feature of P. formosa.

Fn Generations: This is a shorthand to help describe what generation is being described. The n is replaced with the number of the generation. So, the first generation is F1 the second is F2, et al.

One possible explanation for this is that P. formosa is the result of a multi generational hybrid between the various hosts. This theory has its problems as well. For the most part F₁ hybrids are infertile, which would make it somewhat difficult for an F₂ generation to occur. However, it is also often the case that only females of the F₁ generation are infertile, with the males still being able to pass on their genetic material. This would allow for a further mixed F₂ generation and could have resulted in P. formosa. The hybridization theory for the origin of P. formosa has its strengths as well. One is that it would explain how such a similar looking animal could appear at such a high level of life. Also, if this hybridization theory is true, it would provide another possible way that P. formosa has managed to escape Muller's ratchet.


Desirability for Genetic Research

The livebearer P. formosa is desirable in genetic research for a variety of reasons. One of the main reasons is that there is a possibility to have a very specific group of genes in a significantly large number of easy to attain and maintain animals. One "clone,"

Clone: In this context, a group or population of clones. A group of individuals with practically identical genetics.

or lineage of P. formosa that has a common, recent ancestor, is much more genetically uniform than a strain of lab rats that have been specially bred to have similar genetic structures². This means that one clone of P. formosa is more likely to respond similarly to treatments than almost any other species that is available for laboratory research as there are fewer variables that are out of the control of the test administrators. This can lead to more accurate reporting of what is a result of a particular treatment and what is a development of a trait that is genetically encoded into the animal. Some examples of where these fish have been used in order to identify the effect of an external influence on the growth of animals cells is when they have been used in order to identify the damage caused by UV light and ionizing radiation. They have also been used to show how exposure to asbestos affects cell growth and reproduction.

Another way in which P. formosa is influencing genetic research is that triploid individuals and clones have been identified in the wild. This introduces the possibility of an entire populace taking on a triploid genetic trait with no extraordinary disadvantage. This is tied into there being an integration of genetic material from the sperm donating species despite the eggs already being at a diploid state. One of the major cases of interest where triploid individuals exists is in the male P. formosa. All male P. formosa are triploid individuals, leading to the thought that in a clone that consists of triploid individuals, it is possible that males could be a somewhat frequently occurring phenomenon. These males can produce sperm, and are capable of reproduction only through inducing parthenogenesis in female P. formosa. This leads to the thought that if a triploid clone was isolated from its host, there is a chance of a bisexual species being formed where there was no mixing of genes between the two genders.

There is another case of genetic integration besides the case that results in 100% triploid individuals and that is the integration of microchromosomes,

Microchromosomes: "Small, centromere-bearing, supernumerary chromosomes that are an addition to the diploid set of 46 chromosomes."5 An extra bit of genetic information.

This results in a type of evolution that is similar to sexual reproduction, but is closer to the exchange of DNA that occurs in bacteria in that instead of there being a broad, haploid exchange

Haploid: A cell that has only half of the normal genetic information. Usually used in sexual reproduction.

there is a small scale addition of new genetic material. What is unique about this case is that the new genetic material does not replace any of the existing genetic material, but is a supplement to what a clone of the mother would have had. Another extraordinary aspect of this addition of new genetic material is that it is not uncommon for the new genetic material to express itself in the fish. This means that a previously 2n=46 clone could parent a 2n=48 clone that is stable and exhibits the same sexual traits as the 2n=46 clone, but can also exhibit the new traits that have been added. Now whether this clone is a new species is debatable as there is no way in which a mating test could be done. Typically they are still referred to as the same species as the vast majority of their DNA is same and the change came in one generation and made no alteration to the reproductive capacities of the fish. One thing of interest to those studying the effect of microchromosomes on P. formosa is that a clone with one microchromosome is a stable individual and population, but if more than one microchromosome is integrated into the genetic code, the new clone is unlikely to survive.


Importance in Clonal research

P. formosa are also important in discovering where nature ends and nurture begins, as well as how specific the genetic code is in dictating the appearance of the individual. One study that addresses this is the investigation of the pattern in which pigments appeared on a clone of P. formosa that has incorporated a microchromosome that causes the development of pigment filled tumors in about 5% of the fish in the clone. The surprising thing is that the development of these tumors is widely variable, with no one set pattern taking hold in the genetically uniform individuals.⁶ This observation tends to lead to the conclusion that the grip of genetics on phenotypical characteristics is variable and that it is more of a general control than a specific control. Basically this means that the genetic code of a creature does not determine exactly how every cell is going to develop, but that it determines what is needed in an area of the organism and then directs the formation of an object to fulfill that need. In this example the genetic need is for a colored tumor to form in the fishes skin, and so cells are directed to do that, but the individual cells are not directed as to whether or not they are part of the pigment. The forming of the pattern is for the most part random.


The Possible Application of Muller's Ratchet and Methods in which P. formosa Overcomes the Limitations Implied by the Ratchet

First off, a brief definition of Muller's ratchet. Muller's ratchet says that in an asexual population, deleterious mutations

Deleterious Mutation: A mutation that is harmful to a population. A change that makes survival less likely.

will build up over time as there is no chance of the trait not being passed on, like there is in sexual reproduction where only half of the genetic code is passed on at any one time. This means that once a defect exists in the population, there is no way to go back to the original genetic code. This is the origin of the name, as like a ratchet, there is no going back once something has been changed. This means that over time the population as a whole will degrade to non-existent within approximately 10⁴-10⁵ generations. There is also an alternative theory for symbionts and parasites that says that over time there may evolve enough self serving mutations as to make the parasite or symbiont too efficient resulting in the decimation of the host, possibly resulting in the decline of two species.⁷ With P. formosa, this theory seems to have broken down somewhat, the reason for which will be explored shortly. This species is estimated to be around 100,000 year old, with an estimated 300,000 generations having gone by. This is significantly more than Muller's ratchet would suggest. Some possible reasons for this are that although classified as an all female species that reproduces by gynogenesis, microchromosomes are incorporated into the genome from time to time, allowing a small degree of genetic variability to occur. There is also genetic variability brought into the genome through naturally occurring mutations. This allows for some natural selection for the most resilient mutation to out compete and out survive the other variations. One possible example of this is the aggressive mating behavior of P. formosa compared with its sexual hosts. While the females of the various hosts tend to rarely specifically pursue a male of their species, P. formosa is constantly seeking out the males that are needed in order to have gynogenesis occur. Another stabilizing factor that has occurred with P. formosa and its hosts is that females of the hosts copy the mate choice of the female P. formosas. This gives the males that mate with P. formosa an advantage in that their progeny are more likely to survive as there are likely to be more of them.⁵ Another stabilizing factor is that a constant mating to females from males can lead to exhaustion and death of the female. In areas where P. formosa mixes with its hosts, the host females are less frequently pursued by the males of the species as there are more females with which they can mate.⁸ This makes it beneficiary for both species as the female hosts are more likely to live out their lives and the female sexual parasites are able to reproduce regularly.

Perhaps one of the least provable theories for the delay of Muller's ratchet is that if P. Formosa is a hybrid in origin, there still might be this same hybridization continuously ongoing. If this were the case, then Muller's ratchet would have virtually no effect on the population of P. Formosa because there would always be new forms coming about with a genetic code that had been evolving at the much faster rate of sexual reproduction and then becoming the asexual, gynogenetic species.

These are all reasons why the asexual Poecilia formosa is a unique example of reproduction through gynogenesis, as well as a basic outline of why this is an important thing in the world of science today. This one animal, this one little fish has changed in a way that seems to defy the natural law of survival of the fittest, but it continues to strive, creating a system of interdependency in its wild habitat while at the same time providing keys to the process of evolution. This paper is just a brief overview of why it is an important organism for fields of study such as genetic research, clonal research, and why it is an ideal example of asexual species eluding the inevitable end that Muller's ratchet spells out for them.



1. Journal of Biogeography, 29, 1-6.
Biogeography of the Amazon molly, Poecilia formosa

2. ALA Special Puplication #3
The Amazon Molly, Poecila formosa

3. Natl Cancer Inst Monogr. 1984 May;65:45-52
The Amazon molly, Poecilia formosa, as a test animal in carcinogenicity studies: chronic exposures to physical agents.

4. Cytogenetic and Genome Research, Vol. 91, No. 1-4, 2000
Unusual triploid males in a microchromosome-carrying clone of the Amazon molly, Poecilia formosa

5. Cytogenetic and Genome Research, Vol. 80, No. 193-198,1998
Dispensable and indispensable genes in an ameiotic fish, the Amazon molly Poecilia formosa

6. Cancer Research, Vol 57, Issue 14 2993-3000, Copyright © 1997
Susceptibility to the development of pigment cell tumors in a clone of the Amazon molly, Poecilia formosa, introduced through a microchromosome

7. The American Naturalist, vol. 156, no. 4, October 2000
Accumulation of Deleterious Mutations in Endosymbionts: Muller's Ratchet with Two Levels of Selection

8. Behaviour, Feb2001, Vol. 138 Issue 2, p277, 10p
Sexual Harassment as a Cost for Molly Females: Bigger Males Cost Less
 

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