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Male genital diversity gets a rethink: female shapes, ecological factors may explain fast evolution.

Crazily diverse shapes of male genitals across the animal kingdom--from curlicues and Y-tubes to multiknobbed, tendrilly whatsits--may evolve faster than any other animal structures. Biologists have spent more than a century discussing how to explain such fast and extreme variation.

Now it's time to search for explanations in two overlooked places: the female side of sex and the vast variety of places where animals live, researchers proposed in several talks.

Figuring out why male genitals of a species often differ sharply from even its closest relatives' involves basic, big ideas in biology, said Brandon Moore of Sewanee: The University of the South in Tennessee, who coorganized a symposium on male genital diversity. Species arise, flourish or fail depending on whether animals mate and produce offspring or not. "This is where the rubber meets the road in Darwinian evolution," Moore said.

Females supposedly don't show such variety. But that notion rests mostly on previous generations of biologists eyeballing female genitalia or taking simple measurements, said Patricia Brennan of Mount Holyoke College in South Hadley, Mass. "We like to measure length and width," she said. But maybe what matters in male-female interactions are female structures' curvatures, slopes and ratios.

Revisiting female anatomy with modern methods is what Sarah Mesnick of the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, Calif., and her colleagues are doing. Many whales and other cetaceans have "odd and unusual vaginal folds," Mesnick said. Her student Dara Orbach's blue plastic cast of the interior cavity of a harbor porpoise vagina shows broad, slanted-sideways valleys left by the drapery of deep folds in the cavity wall. The team is analyzing variations in such hard-to-even-name shapes that earlier studies missed. More sophisticated understanding may help explain what folds do and whether they have an evolutionary impact on male anatomy.

Genetics suggest strong links between his and hers shapes. For some fruit flies at least, the same gene, called Poxn, has a major influence on shape in a feature of the genitals of each sex, said Eden McQueen of the University of Pittsburgh. Pittsburgh colleagues in the lab of Mark Rebeiz have identified a network of genes, including Poxn, that controls the shape of a stout nubbin called a posterior lobe on male Drosophila genitals. By putting different versions of Poxn into otherwise genetically identical flies, the researchers found that the gene controls not just a male lobe, but also the shape of the oviscapt pouch, a little pocket on female genitals. Altering the shapes of the lobes and pouches changed the length of time flies actually spent copulating.

Thus evolution of shapes can get complicated. The best shape for one sex may not create the best for the other. But because of the shared genes, changing one means changing the other.

Genetics aren't the only way to look for links between the evolution of male and female sex organs. Michael Lough-Stevens of the University of Southern California and his colleagues focused on mammals' mystifying penis bone (baculum) and clitoral bone (baubellum). What benefit they offer is unknown, said Matthew Dean, Lough-Stevens' adviser.

Lough-Stevens is annotating a genealogical tree with what he can find in the scientific literature about which mammals have these bones. This tree may give hints about what forces drove the bones' evolution and how their histories connect.

Of the 128 species or groups on his tree so far, 111 have both bones. In gray squirrels, the male and female bones both look like asymmetric alien ice cream scoops only millimeters long. Sometimes, though, the two bones look nothing alike in the same species. Lough-Stevens showed pictures of a wavy cylinder of bone more than 60 centimeters long from a walrus penis compared with a ragged squiggle of bone only about 5 millimeters long from a walrus clitoris.

Ten species have just a baculum, and the rest, including humans, rabbits and hedgehogs, have neither. So far, Lough-Stevens hasn't found mammals with the female bone but not the male one. He wonders whether lineages that lose genital bones tend to lose the female's first.

Female anatomy has not been the only overlooked topic. Animals live in wildly diverse places, and ecology could explain some divergent anatomy, said Brian Langerhans of North Carolina State University in Raleigh.

He has found links between male fish anatomy and environmental diversity. Gambusia mosquito fish living among predators in the Bahamas tend to grow smaller sperm-delivery organs (gonopodia) than males in safer waters. Females prefer the bigger size, but it's a disadvantage during bursts of escape swimming.

Even human changes to the landscape can affect animal genital shape, said Justa Heinen-Kay, Langerhans' student at NC State. Roadbuilding in the Bahamas has blocked some waterways that once connected to the sea. Male mosquito fish in these closed-off waters no longer contend with big predators cruising in from the sea. And the tips of these mosquito fish gonopodia have widened somewhat. That change may reflect tranquil circumstances that allow males to rely more on female cooperation than just speed in transferring sperm, Heinen-Kay and her colleagues reported in Evolutionary Applications in 2014.

Thus highway planning could also join the list of overlooked sources of variety in the mystery of male diversity.
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Title Annotation:LIFE & EVOLUTION
Author:Milius, Susan
Publication:Science News
Geographic Code:1USA
Date:Feb 6, 2016
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