The ancestor gene, however, showed no such bias. INS元 interacts with a receptor called LGR8 and has been shown to be critical to testicular descent, while RLN3 interacts with a related receptor called LGR7, which has been implicated in mammary gland and nipple development. Hsu's group found that two relaxin family members, RLN3 and INS元, arose from a common ancestor. "We really wanted to understand how these processes evolved," said Hsu, "but we couldn't turn to fossil records for clues because soft tissues, like the testes and nipples, rapidly decompose." When the platypus genome became available for study, Hsu was eager to compare the sequence of their relaxin genes with that of relaxin family members from fish, birds, marsupials and placental mammals. He and his colleagues study a family of genes called relaxins that are involved in a variety of mammal-specific biological processes including nipple development and cervical softening, as well as testicular descent. Hsu wasn't always interested in platypuses.
Although the platypus resembles reptiles in that it lays eggs, it resembles mammals because it secretes milk through the skin for its young and maintains a warmer body temperature. The word, which means "one opening," refers to the fact that the animal defecates, urinates and lays eggs through a single canal, called the cloaca. The platypus belongs to a very small group of mammals called the monotremes.
Evolutionary biologists believe that the adaptation allowed these mammals to have the higher core temperatures necessary for quick reflexes and fast movement. Although, like reptiles, the platypus keeps its testes near its kidneys throughout life, its genome bears the imprint of changes that eventually allowed most other male mammals, including humans, to store their reproductive organs in cooler temperatures outside the body. In appearance, the platypus is a beaverlike animal with webbed feet and a rubbery beak similar to a duck's. Also on May 8, Nature published a paper on the sequencing of the platypus genome. Hsu's research was published online May 8 by Genome Research along with several other papers devoted to findings made possible by the sequencing of the platypus genome. "The platypus serves as a 'bridge' animal between nonmammals like birds and reptiles, which maintain their testicles in their body cavity, and placental and marsupial mammals, which hold their testes in an external scrotum." "Testicular descent is a very specialized process that required the evolution of specific genes," said Sheau Yu Teddy Hsu, PhD, assistant professor of obstetrics and gynecology and senior author of the study. Understanding the molecular mechanisms behind this adaptive process may help physicians understand why the testes of about 30 percent of premature boys fail to descend to the scrotum properly. Moving the testicles' heat-sensitive cargo, the sperm, to an outer pouch called the scrotum allows it to be stored in a cooler environment. The information enabled the Stanford researchers to trace the evolution of two genes involved in moving the testicles away from the warm core of the body during the development of most mammals. The newly sequenced platypus genome, published concurrently with the Stanford research, provides a wealth of data about the unique egg-laying mammal. Now researchers at the School of Medicine have turned to the strange animal to understand the genesis of an evolutionary tour de force that led to a reproductive advantage possessed by nearly all of today's mammals.īiologists have debated the origins of the Australian animal for more than 200 years. The odd-looking platypus is often cited as proof that God has a sense of humor.