Category Archives: sex

On Spermatozoic-Evolution

I recently listened to an episode of RadioLab concerned with the subject of sperm. It was highly enlightening, as most of their programming is, in my opinion, and it turned me on to one concept in particular that I found of particular interest. In general amongst our close animal relatives, promiscuity is the rule; approximately 3 percent of mammalian species are considered monogamous. One predicted result of this behavioral ubiquity is the specific evolution of sperm, for if male genes are to be carried on, an individual’s sperm must compete with the sperm of others inside the female for the right to fertilize her egg(s). In fact, it has been known for some time that evolutionary selection will operate on sperm whenever access to a female’s eggs is contested by sperm from more than one male1. Furthermore, those who speculate about the subject speculate that such competition should yield larger sperm, based on the paired assumptions that larger sperm are faster, and faster sperm are more likely to fertilize an egg.


A study published in the Proceedings of the National Academy of Sciences has shown that female promiscuity does in fact, lead to the evolution of faster sperm in 29 closely related species of cichlid fishes of Lake Tanganyika, Africa. These fish in this lake are of particular interest to evolutionary researchers and theorists because the lake is large enough to constitute several environments – thus it harbors several closely related but distinct species of cichlids – and because of certain “explosive speciation events2,” the relationships amongst these species is very well documented.


These researchers scored each species, assigning them a number according to their “sperm competition rank” (see table above). Which strongly predicted the speed of those species sperm (see table, below).

This research is quite intriguing because it represents an example of behavior feeding back on evolution. The effects of behavior on evolution are fascinating because such phenomena must have played a significant role in our own evolution, and continue to be perhaps the most important determinant of our biological fate.

References:
1. Parker GA. Sperm competition and its evolutionary consequences in the insects. Biol Rev 45: 525–567, 1970.
2. Fitzpatrick JL, Montgomerie R, Desjardins JK, Stiver KA, Kolm N, Balshine S. Female promiscuity promotes the evolution of faster sperm in cichlid fishes. Proc Natl Acad Sci U S A 106: 1128-32, 2009.

On Male Versus Female (bugs)

Male fruitflies know what their ladies like. They court them with the dulcet tones of their wings themselves. It has been observed that one behavior that males of the species Drosophila Melanogaster can engage in that appears to increase his likelyhood of copulating with a female, is the rhythmic production of sound by wing-vibration; that is if she finds his performance to be sufficiently virtuosic. This behavior is controlled by a central pattern generator (CPG), a group of neurons that, more or less, independently generates the activity corresponding to this motion. Think of this as a sort of reflex, somewhat akin to walking at a constant pace over a completely flat surface in that it is extremely stereotyped and largely automatic.

Nature Magazine

One question this has raised in researchers minds is why this behavior is limited to males. A new study demonstrates that the females have this ability, but they simply suppress it. Apparently the neurons that kickstart the motor of the central pattern generator are simply not active in females. In order to get these cells going, Jai Y. Yu and Barry J. Dickson used a technique involving the manipulation of ion channels with light.

Every neuron’s excitability is mediated by the opening and closing of small pores (ion channels) in their membranes which allow charged ions to flow in and out. The regulation of this permeability or conductivity of the membrane determines how excited the neuron can become. Put another way, if a bunch of little doors burst open that allow positively charged ions to flow into the cell, those ions flood in, raising the voltage of the cell, putting it in an excited state. It turns out that certain species of algae have specialized ion channels that are of some use to scientists. Usually ion channels are opened in response to neurotransmitter or enzymatic signals, but these plants have ion channels that are directly “gated” (opened or closed) by impinging light. What Yo & Dickson have done is to exploit a technique in which they borrow these channels and put them into specific cells using molecular genetic techniques, thus allowing them to selectively excite specific ion channels in specific neurons with bursts of luminance.

In this way, they were able to observe that, through the excitation of neurons controlling the CPG, the ladies are able to produce these courtship songs as well, they just didn’t know they had it in them.

On Sex

(IMAGEBROKER/ALAMY)

Sex changes things. The act of intercourse between two people inevitably changes some aspect of a pair’s interaction. This may be a dual effect, or it may simply manifest from modifications of the behavior of the individuals.

As human beings, we tend to focus more on the psychological or sociological implications of coupling, but there are also biological ramifications that can be independent of or intertwined with cognitive facets of sexual congress. It is thought that many of the behavioral effects have evolved to increase the likelihood that a union yields offspring, or simply to increase the reproductive success of one of the putative parents.

A recent article appearing in Nature highlights one striking example of such a convolved effect, the so-called sex peptide and its cognate receptor in the fruit fly Drisophila Melanogaster. The work, from the lab of Barry J. Dickson at the Research Institute of Molecular Pathology in Vienna, both identifies the receptor and the sites at which it can be found in the fly’s body1.

It had been known for some time that sex peptide (found in the male’s seminal fluid), acts on the female in such a way that she is more likely to lay eggs, and less likely to copulate again. This has the effect of increasing the male’s chance of producing heirs; keeping his sperm from being diluted by competing sires, and increases the number of potential progeny. Of course, this has a cost for the female, as she is now less likely to find a more-fit partner.

There are also many other examples worth mentioning (these were brought to my attention by an excellent comment on the paper, also in Nature2):

In mammals, intercourse alters the environment of the reproductive tract at an immunological level, increasing the probability of fertilization and implantation, perhaps explaining the increased likelihood that a female can contract a urinary tract infection.

The males of many species (avian, reptile, rodent) will simply guard a female post-coitally to prevent her from finding a (potentially) more desirable mate.

In snakes and some insects, sex can actually lead to changes in the pheromones that a female produces, rendering her less attractive to other possible suitors.

If a female rodent is exposed to the pheromones of a dominant male, she develops a sort of taste for power, making her more likely to reject a subordinate gentleman caller.

Female jewel wasps have an even simpler version: their response to male sex-pheromones changes from attraction to aversion after copulation. Similarly, and again in the fruit fly, females will actively turn-down new beaus, running away from newly encountered males and even kicking them.

While there is no known human homologue of the genes identified by the study I mentioned above, all of these examples beg the question of what forms of this sex-mind-control might be at work in our species. It’s also interesting to consider that all the thus-far identified effects are of males on females, could there also be some method that women have of controlling the men as well? In any case, knowing that our very biology might change as the result of certainly make me wonder what might at work in my brain & body after a roll in the hay.

References

1. Yapici, N. et al (2008) A receptor that mediates the post-mating switch in Drosophila reproductive behaviour. Nature 451, 33–37

2. Griffith L.C. (2008) Neuroscience: Love hangover. Nature 451, 24-25