Think the eye defies evolutionary theory? Think again

The compound lens of the insect eye

Win for Darwin

When Darwin proposed his theory of evolution by natural selection, he recognized at the time that the eye might be a problem. In fact, he even said it was “absurd” to think that the complex human eye could have evolved as a result of random mutations and natural selection. Although evolution remains a fact, and natural selection remains a theory, the human eye now has some solid evolutionary precedence. A group of scientists that has established a primitive marine worm, Platynereis dumerilii, as a developmental biology model has found that it provides the key to the evolution of the human—and insect—eye.

Multiple events in eye evolution, or only one?

The divide over the eye occurred because the insects have the familiar compound-lens—think how fly eyesight is depicted—and vertebrates have a single lens. Additionally, insects use rhabdomeric photoreceptors, and vertebrates have a type known as ciliary receptors. The rhabdomeric receptors increase surface area in the manner of our small intestine—by having finger-like extensions of the cell. The ciliary cells have a hairy appearance because of cilia that pop outward from the cell. A burning question in evolutionary biology was how these two very different kinds of eyes with different types of photoreceptors evolved. Were there multiple events of eye evolution, or just one?

Just once?

P. dumerilii work indicates a single evolutionary event, although the usual scientific caveats in the absence of an eyewitness still apply. This little polychaete worm, a living fossil, hasn’t changed in about 600 million years, and part of its prototypical insect brain responds to light. In this system is a complex of cells that forms three pairs of eyes and has two types of photoreceptor cells. Yep, those two types are the ciliary and the rhabdomeric. This little marine worm has both kinds of receptors, using the rhabdomeric receptors in its little eyes and the ciliary receptors in its brain. Researchers speculate that the light receptors in the brain serve to regulate the animal’s circadian rhythm.

How could the existence of these two types of receptors simultaneously lead to the evolution of two very different kinds of eyes? An ancestral form could have had duplicate copies of one or both genes present. Ultimately, if the second copy of the rhabdomeric receptor gene were recruited to an eye-like structure, evolution continued down the insect path. But, if the second copy of a ciliary cell’s photoreceiving gene were co-opted for another function, and the cells were ultimately recruited from the brain for use in the eye, then evolution marched in the vertebrate direction.

All of the above is completely speculation, although this worm’s light-sensitive molecule, or opsin, is very much like the opsin our own rods and cones make, and the molecular biology strongly indicates a relationship. It doesn’t completely rule out multiple eye-evolution events, but it certainly provides some nice evidence for a common eye ancestor for insects and vertebrates.

Note: This work appeared in 2004 and got a detailed writeup at Pharyngula.

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Can rain make buffalos have boys?

African buffalo shift sex ratios with rain

African buffalos (Syncerus caffer) have more males during the rainy season in Kruger National Park, and it’s not just a random accident of fate. Researchers have found that specific sequences on the Y chromosome are correlated with seasonal differences in birth sex ratios in the buffalo population.

X sperm vs. Y sperm

Does that mean that rain somehow makes buffalos have more boys? Not directly. Instead, it may come down to a DNA-level battle royale involving the Y chromosome. Sometimes, sperm carrying the Y win the race to the egg, while at other times, X-carrying sperm are the victors. These times correlate with higher frequencies of certain sequences, or haplotypes, of the Y chromosome occurring in the population, with one sequence being much more common during the rainy season, when more males are born.

Selfish genes gone rogue

The investigation suggested the existence of a suppressor of Y chromosome success acting during the dry season, when females birthed more females, and a distorter in favor of Y chromosome success in the rainy season, when more males are born. The distorter may shift meiosis in favor of the Y-carrying sperm or disrupt survival of X-carrying sperm. Interestingly, distorters are not considered to act for the benefit of the individual carrying them and are considered “selfish genes.” Suppressors…well…suppress the distorters. The authors refer to these apparent Y chromosome suppressor/distorter regions as sex-ratio, or SR,  genes.

Dry season not a good sperm season

They also noted that during the dry season, buffalo didn’t make as much sperm, and the sperm they did make weren’t as frequently normal looking or very good swimmers. They hypothesize that semen quality may interact with the decreased availability of food in the dry season, leading to drop in Y haplotypes associated with a male-biased sex ratio. The investigative team, whose lead author, Pim van Hooft, is based at Wageningen University in The Netherlands, also suggested that the SR genes may be present in other species, adding a new dimension to the increasingly complex mechanisms of sex ratios in mammals.

For your consideration

1. Sex determination in vertebrates happens in a number of different ways. Some mechanisms don’t involve sex chromosomes at all but instead rely on environmental cues. Find an example of a species that uses environmental cues to determine sex. How can an environmental trigger be similar to a chromosomal trigger as a sex determinant? How do they differ?

2. Many species have life history strategies that involve adjusting sex ratios. What are possible explanations can you find to explain how adjusting sex ratio might benefit a species? How might it be a potentially dangerous gamble?

3. Distorters in general appear to be doing their host individual no favors. Given that fact, what is one explanation for the existence and persistence of suppressors of distorters?

Flying drunk no problem for bats

Drunk New World bats fly fine under the influence

People can’t do it. When we drink, alcohol impairs all kinds of functions, including our ability to drive or walk a straight line. Bat researchers in work published in the online open-access journal PLoS ONE hypothesized that the same rule would apply to bats: the frugivorous (fruit-eating) types often encounter fermented fruits, meaning that frequently, a meal for a bat comes with the alcohol equivalent of a dry martini.

Sonar unaffected

And the humans–not for the last time–were wrong. Bats flying under the influence of a blood alcohol measuring three times the human legal limit maneuvered just fine in their human-imposed drunk tests. The test consisted of plastic chains suspended from the ceiling, requiring the bats to make their way around and through without a collision. Whether they’d imbibed sugar water or grain alcohol, the world’s only flying mammals performed equally well.

Bats may build up a tolerance

Not all bat species have this capacity. It seems that bats, like people, may vary in their alcohol tolerance. In addition, bat species like the New World bats in this study that encounter fermented fruits all the time may have a better tolerance for alcohol than bats who imbibe only occasionally. Old World bats, it appears, are less able to hold their liquor compared to their New World, daily imbibing cousins.

Alcohol: a previously unidentified force of natural selection?

Humans may have long been aware that alcohol can drive certain choices. And now, the bats may confirm that. According to the study authors, sensitivity to ethanol may have determined which bat species developed where. Just as types of fruit may have influenced the speciation of bats, the bat ability to tolerate–or not–ethanol may also have affected bat adaptive radiation.

Ideas for questions

Bats navigate by sonar, while humans rely primarily on inputs including vision to maintain balance and walk a straight line. Do you think that this difference might help explain why these New World bats don’t show the effects of alcohol in their navigation? Why or why not?

The paper refers to the bat “adaptive radiation.” What is an adaptive radiation, and what are the conditions that are required for one to occur? How did bat speciation exemplify this process?

Other frugivorous or omnivorous species encounter fermented foods, as well. One hypothesis, the Drunken Monkey hypothesis,  is that the smell of fermenting fruit drove primate evolution. Can you find other research describing the influence of ethanol on animals?

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