What Drove The Evolution Of Male Sharks' Genitals?

Scott Bennett, Martin J. Cohn, Katherine O'Shaughnessy, Nature Comms, 2015. Via Science.

There is no shortage of fascinating genitals in the animal kingdom, which can often make those of our own species seem rather dull in comparison. Male cartilaginous fishes, such as sharks and rays, for example, possess a pair of pointed “penises” called claspers. These specialized organs are actually elongations of the pelvic fins and serve to assist the transfer of sperm to the female during copulation.

But it is not simply the funky appearance of these structures that has intrigued scientists; since both male and female chondrichthyans (sharks, skates and rays) possess pelvic fins, why is it that only males undergo this modification? That was the central question a group of researchers from the University of Florida endeavored to find the answer to, and they believe they have come across an explanation. According to their findings, which have been published in Nature Communications, regulation of a result of a well-known genetic pathway, called Sonic hedgehog (yes, really), by sex hormones could be driving the development of claspers.

To make this discovery, the researchers reared little skate (Leucoraja erinacea) and compared the pelvic fins of males and females as they developed. They found that the immature fins, or fin buds, are indistinguishable early on in development, but males undergo expansion during a late phase of fin development. After making these morphological observations, the researchers decided to see what was going on at the genetic level, so they compared gene expression patterns over time as the embryos developed.

In particular, they were interested in the Sonic hedgehog (Shh) gene pathway since, amongst other things, this drives the development of vertebrate appendages, Science explains. They discovered that the Shh circuit was active in the pelvic fin area of males for approximately a month longer than females. Probing this further, the researchers switched off Shh in the pelvic fins of males, which stunted clasper development. Furthermore, when they prolonged its activity in females, they began to develop claspers like males would.

They then began digging deeper to find out what is keeping this circuit active for longer in males, which led them to the discovery that the sex hormone, androgen, controls the male-specific pattern of Shh in pelvic fins by regulating another molecule called Hand2. Hand2 is a transcription factor, a protein that sticks to specific genes and switches them on or off. More specifically, this protein activates a stretch of DNA which acts to enhance the expression of Shh.

The researchers therefore concluded that, at some stage in their evolutionary history, the gene circuit for appendage development of chondrichthyans evolved to be regulated by the sex hormone androgen, which ultimately prolonged its activity in males and thus drove the development of claspers in males.

[Via Nature Communications and Science]

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