wes_von_papineäu
Our Roving Correspondent
SCIENCE NEWS (Washington, DC) 12 April 08 Vol. 173, No. 15 , p. 232 All in the Family - For some animals, the ideal mate is a brother, sister or cousin (Ewen Callaway)
In late March, as winter unclenches its frigid grip on upstate New York, a spotted salamander's thoughts turn fleetingly to love. After early spring rains soak the forests where the salamanders live, thousands of the slimy little creatures descend on small vernal pools for the amphibian equivalent of an orgy.
"It's sort of a frenzy," says Kelly Zamudio, an ecologist at Cornell University who studies the five-day ritual. "All these males are competing with each other and nudging each other and putting down sperm as quickly as they can."
Spotted salamander sex, it turns out, is an evolutionary Easter egg hunt. Males lay scores of sperm-filled pouches onto the leaves and twigs that litter the pond floor, while females pluck away the sperm to fertilize their eggs. The pouches - called spermatophores - look like little soccer trophies, "but made out of jelly," Zamudio says. A female often collects more than a dozen trophies left by various males. Then, "she walks away from this aggregate of males who are going crazy putting down their spermatophores. She's got everything she needs."
Zamudio and former student Chris Chandler wanted to know which males passed on their genes most successfully. Since spotted salamanders don't copulate, females have no direct way to assess the potential fathers of their children. So the scientists analyzed DNA collected in the field from males, females and larvae and came to a surprising conclusion.
"She seems to be fertilizing her eggs with gametes of animals that are a little bit more closely related to her," Zamudio says. In other words, the salamanders are inbreeding.
In fact, many animals prefer to mate with their brothers, sisters and other relatives. Humans, too, might even favor kissing cousins. Zamudio and Chandler published their findings last month in Molecular Ecology. Other researchers have uncovered the same surprising trend in species of fish, birds and beetles.
Long stigmatized because of the birth defects that often afflict children of closely related couples, inbreeding sometimes comes at a high price. Humans and other animals have two versions of most genes—one from mom and one from dad. Inbreeding ups the chances that a child will inherit two versions of a disease-causing gene. The son of first cousins, French artist Henri de Toulouse-Lautrec suffered from an inherited form of skeletal dysplasia that left the artist with short legs and weak bones. And consanguinity among royal families in 19th century Europe contributed to a high incidence of the blood disease hemophilia. England's Queen Victoria was a carrier of the disease, and hemophilia scourged the royal houses of Spain and Russia, as well.
But inbreeding can have its perks, says Patrick Bateson, a biologist at the University of Cambridge in England. Many organisms might have slight genetic tweaks or adaptations tuned to their local habitats, and too much genetic mixing with outsiders can dilute these adaptations, he says. For instance, horticulturalists self-fertilize plants to preserve genes that confer advantages like pest resistance. "What you see then is a kind of trade-off between the costs of inbreeding and the costs of outbreeding," Bateson says. "It's not that inbreeding as such is beneficial. It's getting the balance between inbreeding and outbreeding that is important."
Zamudio's analysis of spotted salamander mating hints at such a trade-off. Comparing the DNA of fathers, mothers and larvae, she and Chandler found an intermediate amount of inbreeding—at the level of first cousins, on average. Despite having hundreds of possible mates to choose from, females tended to fertilize their eggs with sperm from related males. The females don't choose spermatophores at random, and they certainly don't avoid their kin, she says.
One question Zamudio hopes to answer is how female salamanders tell a cousin's spermatophore from the hundreds of others. She says the females might detect some genetically determined pheromone wafting off the sperm. Another possibility is that sperm from related males outcompetes sperm from less-related males to fertilize eggs.
She also doesn't know why her spotted salamanders appear to prefer to inbreed. The amphibians might use inbreeding to hold onto local genetic adaptations, she says. "If a male moves from one pond to a neighboring pond, he may carry with him genes that are just slightly deleterious for the pond where the female lives—just slightly off—so she would benefit from actually choosing males from her own pond because they're best adapted." Zamudio has noted tiny differences among the DNA of animals from different ponds.
http://www.sciencenews.org/articles/20080412/bob8.asp
In late March, as winter unclenches its frigid grip on upstate New York, a spotted salamander's thoughts turn fleetingly to love. After early spring rains soak the forests where the salamanders live, thousands of the slimy little creatures descend on small vernal pools for the amphibian equivalent of an orgy.
"It's sort of a frenzy," says Kelly Zamudio, an ecologist at Cornell University who studies the five-day ritual. "All these males are competing with each other and nudging each other and putting down sperm as quickly as they can."
Spotted salamander sex, it turns out, is an evolutionary Easter egg hunt. Males lay scores of sperm-filled pouches onto the leaves and twigs that litter the pond floor, while females pluck away the sperm to fertilize their eggs. The pouches - called spermatophores - look like little soccer trophies, "but made out of jelly," Zamudio says. A female often collects more than a dozen trophies left by various males. Then, "she walks away from this aggregate of males who are going crazy putting down their spermatophores. She's got everything she needs."
Zamudio and former student Chris Chandler wanted to know which males passed on their genes most successfully. Since spotted salamanders don't copulate, females have no direct way to assess the potential fathers of their children. So the scientists analyzed DNA collected in the field from males, females and larvae and came to a surprising conclusion.
"She seems to be fertilizing her eggs with gametes of animals that are a little bit more closely related to her," Zamudio says. In other words, the salamanders are inbreeding.
In fact, many animals prefer to mate with their brothers, sisters and other relatives. Humans, too, might even favor kissing cousins. Zamudio and Chandler published their findings last month in Molecular Ecology. Other researchers have uncovered the same surprising trend in species of fish, birds and beetles.
Long stigmatized because of the birth defects that often afflict children of closely related couples, inbreeding sometimes comes at a high price. Humans and other animals have two versions of most genes—one from mom and one from dad. Inbreeding ups the chances that a child will inherit two versions of a disease-causing gene. The son of first cousins, French artist Henri de Toulouse-Lautrec suffered from an inherited form of skeletal dysplasia that left the artist with short legs and weak bones. And consanguinity among royal families in 19th century Europe contributed to a high incidence of the blood disease hemophilia. England's Queen Victoria was a carrier of the disease, and hemophilia scourged the royal houses of Spain and Russia, as well.
But inbreeding can have its perks, says Patrick Bateson, a biologist at the University of Cambridge in England. Many organisms might have slight genetic tweaks or adaptations tuned to their local habitats, and too much genetic mixing with outsiders can dilute these adaptations, he says. For instance, horticulturalists self-fertilize plants to preserve genes that confer advantages like pest resistance. "What you see then is a kind of trade-off between the costs of inbreeding and the costs of outbreeding," Bateson says. "It's not that inbreeding as such is beneficial. It's getting the balance between inbreeding and outbreeding that is important."
Zamudio's analysis of spotted salamander mating hints at such a trade-off. Comparing the DNA of fathers, mothers and larvae, she and Chandler found an intermediate amount of inbreeding—at the level of first cousins, on average. Despite having hundreds of possible mates to choose from, females tended to fertilize their eggs with sperm from related males. The females don't choose spermatophores at random, and they certainly don't avoid their kin, she says.
One question Zamudio hopes to answer is how female salamanders tell a cousin's spermatophore from the hundreds of others. She says the females might detect some genetically determined pheromone wafting off the sperm. Another possibility is that sperm from related males outcompetes sperm from less-related males to fertilize eggs.
She also doesn't know why her spotted salamanders appear to prefer to inbreed. The amphibians might use inbreeding to hold onto local genetic adaptations, she says. "If a male moves from one pond to a neighboring pond, he may carry with him genes that are just slightly deleterious for the pond where the female lives—just slightly off—so she would benefit from actually choosing males from her own pond because they're best adapted." Zamudio has noted tiny differences among the DNA of animals from different ponds.
http://www.sciencenews.org/articles/20080412/bob8.asp