Friday, March 7, 2014

The Pros and Cons of Mating with an Amazonian Female

By Rachel Wilson

Sex is important, and not just because it produces babies and facilitates social bonding but because it allows for genetic recombination. This allows offspring to be genetically unique which ultimately increases resilience of the species. In turn resilience increases the chances that at least some individuals in a population will be able to cope and adapt to random change in the environment. This is good, as it maintains biodiversity and as a biologist this is something that I think is important.
Genetic recombination is achieved through reproduction. Sexual reproduction results in, arguable, the greatest genetic diversity as two distinct individuals copulate to produce genetically unique offspring. This just so happens to be the type humans are most familiar, mainly because we perform this kind of reproduction. With life being so very diverse on our planet, there has to be other forms of reproduction out there! And no, I don't mean sexual fetishes…

I mean reproduction, or, the production of offspring. Some organisms reproduce asexually, meaning the genetic material inherited by the offspring is from a single parent. This type of reproduction is called parthenogenesis. This doesn’t necessarily mean that the offspring is a clone, although some organisms, mainly microbes, reproduce this way. In fact, some asexually reproducing species require copulation, or mating, to procreate. However, the paternal genetic material ultimately is ‘kicked out’ from the embryo. Talk about being used for sex.
Parthenogenesis occurs when the progeny of an individual only has the maternal genetic material. A subset of parthenogenesis, gynogenesis ‘kicks out’ the paternal genetic material after mating, and therefore only the maternal genetic material contributes to the offspring’s genome. However, unlike parthenogenesis, species reproducing by gynogenesis must mate with a male (Schlupp et al., 1994). The schematic below sums up this nicely. M refers to maternal contributions, while P refers to paternal contributions (Schlupp et al., 1994).

Schlupp et al., 1994 Figure 1

Asexual reproduction implies that all the offspring will be genetically very similar to their parents. As the population is all female, the progeny will be as well (usually). But if it’s an all-female species, then how do they mate with males? It turns out theses females look to their close relatives to mate. The Amazon molly (Poecilia Formosa), a gynogen species, mates with either a male Atlantic molly (Pocecilia mexicana) or sailfin molly (Poecilia latipinna) in order to produce offspring (Marler and Ryan 1997). 
Since, the male contributes no genetic material, I presumed there would be a very strong selection pressure against mating with a female who is literally using you for sex. Males mating with gynogens aren't increasing their fitness by having more offspring because the offspring produced aren’t genetically similar. So I would think that males would preferentially chose not to mate with Amazon mollies, or they would have to be coerced into mating with such an Amazon.

Male Atlantic and sailfin mollies preferentially chose to mate with their corresponding species, although sailfin mollies as less discriminating than Atlantic mollies (Ryan et al., 1996; Gabor and Ryan, 2001). Despite this preference, Amazon molly populations still persist. Therefore there must either be some sort of advantage male Atlantic or sailfin mollies obtain from mating with these females, or the males somehow mitigate the decrease in fitness caused by mating with a heterospecific, or a differing species.

In order to mitigate the cost of decreased fitness of mating with heterospecifics, male sailfin mollies employ two tactics. Firstly, as seen below, male sailfin mollies spent less time with Amazon mollies during the sailfin molly breeding season (Heubel and Schlupp, 2008). Correspondingly, there was a significantly lower proportion of Amazon molly juvenilles in the wild during the sailfin molly breeding season (Heubel and Schlupp, 2008). These figures show that when the availability of receptive sailfin molly females increased, male sailfin mollies ditched their second choice Amazon mollies and started spending time with females that would give them genetically similar progeny thereby increasing the male’s fitness.

Heubel and Schlupp, 2008 Figure 2

Heubel and Schlupp, 2008 Firgure 1

Not only do males’ preferences change by season, sailfin males further mitigate decreased fitness by sperm alterations. When males were exposed to Amazon mollies, they primed less sperm than males exposed to conspecifics, or females of their own species (Aspbury and Gabor, 2004). So sailfin males decreased their sperm viability in the presence of Amazon mollies in order to prevent unnecessary expenditure of energy. So not only do males ditch Amazon mollies during the sailfin breeding season, they also don’t spend as much energy on mating with Amazons as they do with sailfins… men right? Well really what can we expect when some females only use them for their sperm casing?
Even though he’s not that into Amazonians, he, being sailfin mollies, still benefits from mating with them. Females witnessing a male mating become more ‘attracted’ to him, even if that male is mating with an individual of another species. So a female sailfin prefers a mating male sailfin. This preference persists even if that male is mating with an Amazon molly (Schlupp et al. 1994).  Mating, in general, will increase a male sailfin molly’s chance of perpetuating his genetic material because he will be more likely to mate with other females that have observed him mating. Perhaps this is why male mollies haven’t evolved to be completely discriminating in heterospecific mate choices. Even if he is mating with a heterospecific that will produce no genetically similar offspring, the chances of him mating with a conspecific will increase. A mating with a conspecific will increase his fitness so long as progeny is produced.

What could possibly further this behavior corresponds to other males witnessing a mating. Sailfin males copy other male’s mate choices. So if a male sailfin observed another male sailfin mating with an Amazon molly, he is more likely to mate with an Amazon molly was well (Schlupp and Ryan, 1997). It appears there is a positive feedback loop to mate with heterospecifics once an individual has mated with an individual of the opposite sex. This is at least partially the reason why this behavior hasn't been selected against.

It appears the pros of mating with a heterospecific outweigh the cons. Which is important in maintaining the Amazon molly species. Ultimately, Amazon mollies rely on closely related mollies to ensure the species remains extant. If males weren’t as discriminating against Amazon mollies and in favor of their corresponding conspecifics, Amazon mollies would go extinct. If male sailfin mollies chose to mate with Amazon mollies and female sailfin mollies at the same proportions, Amazon mollies would become extinct (Heubel et al., 2009). Luckily for the Amazon mollies, males of the sailfin molly species aren’t so choosy. Otherwise that would mean Amazon mollies would go extinct by sex, or lack there of. Who would have thought death by sex is applicable in the natural world.

Click --> Death by Snu Snu

Aspbury, A.S. and C.R. Gabor. 2004. Discriminating males alter sperm production between species. PNAS 101(45):15970-15973.

Gabor, C.R. and M.J. Ryan. 2001. Geographical variation in reproductive character displacement in mate choice by male sailfin mollies. Proc. R. Soc. Lond. B 268:1063-1070.

Heubel, H.U., D.J. Rankin, and H. Kokko. 2009. How to go extinct by mating too much: population consequences of male mate choice and efficiency in a sexual-asexual species complex. Oikos 118:513:520.

Heubel, H.U., and I. Schlupp. 2008. Seasonal plasticity in male mating preferences in sailfin mollies. Behavioral Ecology 19:1080-1086.

Marler, C.A. and M.J. Ryan. 1997. Origin and maintenance of a female mating preference. Evolution 51(4):1244-1248.

Ryan, M.J., L.A. Dries, P. Batra, and D.M. Hillis. 1996. Male mate preferences in a gynogenetic species complex of Amazon mollies. Animal Behavior 52:1225-1236.

Schlupp, I., C. Marler, and M.J. Ryan. 1994. Benefit to male sailfin mollies of mating with heterospecific females. Science 263(5145):373.

Schlupp, I., and M.J. Ryan. 1997. Male sailfin mollies (Poecilia latipinna) copy the mate choice of other males. Behavioral Ecology 8(1

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