In or out: Platypus genome reveals the truth behind mammalian testes

Keen to unravel the story of mammalian emergence, scientists have sought for answers in monotremes such as the platypus, the laughable but evolutionarily significant fellow that oddly presents the physiological features of both reptile and mammal.
The decoding of the platypus genome has allowed the investigation of a key milestone with regards to mammalian evolution - the separation of sperm from the metabolically heated internal body to within an external scrotum. The platypus has been discovered to be effectively a ‘bridge’ between the branches of phylogeny, as its attributes link the mysterious gap between reptilian male reproductive systems and the development of external testicles in mammals.
Past studies have revealed that the INSL3 gene and its receptor LGR8 are essential in the process of testicular descent during early development, while the gene RLN3 interacts with LGR7 to contribute to mammary gland development. Also concluded was that RLN3 and INSL3 arose from a common ancestor.
Recently, one of the many research groups drooling over the freshly decoded platypus genome found that the platypus lacks a protein which specifically binds to LGR8, thus explaining its reptilian-like internal testes, but does have the specific receptor LGR7. Animal groups earlier in the phylogenetic tree, including fish and reptiles, merely display the ‘ancestor-like genes’ and therefore do not exhibit scrotums or nipples.
Testicular descent is a unique adaptation present in most mammals that allows for the optimal synthesis of sperm below the core body temperature. Such discoveries highlight the genetic changes which have lead to the widespread reproductive successes of mammals sporting external testes.

Written by Jo Taylor

Primary reference - http://www.pr-inside.com/platypus-genome-reveals-evolutionary-path-r577565.html
http://www.genome.org/cgi/content/abstract/gr.7119108v1
http://soer.justice.tas.gov.au/2003/image/559/ilw/p-platypus_m.jpg
More on the fascinating platypus genome and mammalian evolution - http://www.sciencedaily.com/releases/2008/05/080507131453.htm