Friday, March 19, 2010

Sperm Wars

There are as many types of insect mating systems as there are insects themselves, but they can be generalized in a few ways. In terms of mating strategies, or pre-copulatory traits or behaviors, many insects use displays to entice a mate. Such displays may include body size or color comparisons, dances, or fights. Other indicators of suitability may include territory or food with some species offering up nuptial gifts (the quality of which is what is judged).

Just because a mate is selected, does not mean they are the sole contributor of gametes. Terrestrial insects (males) transfer spermatophores, packets of sperm, to the female. Depending on the species, the spermatophore may be alone, in pairs, in groups, stored for a long time, ejected, or mixed. As such, males have their own methods to make sure theirs is the sperm used to fertilize the eggs. Such methods may include mate guarding, extended copulation, flushing/clearing out other sperm, and/or genital plugging. Should rival males each contribute, and those contributions allowed to make a mad dash for the egg, it is straight up sperm competition that takes over - the fastest, strongest swimmers win! Again, these are generalizations and there are oh-so-many variations on these strategies.

Two new studies take a look at the perilous life of sperm. The studies detail sperm competition and how the female can manipulate the situtation to her own ends.

In this corner!!... a population of genetically engineered fruit flies from the U.S.!!

...and in this corner!!... ants and bees from Denmark and Australia!!...

Ok, a little dramatic, I know. So let's get down to business. The fruit flies (Drosophila melanogaster) were engineered to produce fluorescent sperm of different colors. Besides the obvious humor of glowing sperm, the little guys could be seen as they jostled for position inside the female reproductive tract. Typically, the most recent male to mate with the female has the most success in fertilization (80%), but until now it was not known why. A female fruit fly only keeps about 500/1400 sperm from each male she has mated with. After mating with a male, the female transfers this subset of sperm to one of 3 blind-ended tubes branching off of her reproductive tract. Just as a second mating begins she releases sperm from the first mating. Essentially, the sperm from male #2 flushes out the sperm from male #1.

The second study looked at honeybees and leafcutter and army ants, known for their large, complex societies. In these social insects, a single member of the society is responsible for reproduction (the queen) and must have a large supply of sperm to fertilize her eggs. We're talking storing multi-millions of sperm over a long period of time (years or even decades!) and producing millions of offspring in a single lifetime. That storage time creates a lot of opportunity for sperm competition. In these insects, it was shown that the chemicals in seminal fluid aids a male's own sperm but attacks that of his rivals. However, this chemical warfare is only seen in insects where the sperm is held together - in bumblebees, for example, the sperm of different males never meets and so you don't see the presence such chemicals. Although, the study also found that some queens (leafcutter ants) have the ability to suppress the competition - the ring leader so to speak - because large long-term supplies are precious.

So why all the fuss? Mating with multiple males ensures diversity in the offspring. Additionally, the fittest males (and their fit sperm) win out and end up fathering the most offspring. And diversity plus fitness equals good.

Here's the write up in Nature: http://www.nature.com/news/2010/100318/full/news.2010.135.html

The fruit fly paper: http://www.sciencemag.org/cgi/content/abstract/science.1187096v1

The ant/bee paper: http://www.sciencemag.org/cgi/content/abstract/327/5972/1506

(images from pnas, sciencecentric.com, blogs.cas.suffolk.edu and Science mag via Nature mag)
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