Did you know that male cartilaginous fish (sharks, rays and skates) are equipped with two penis-like structures, known as "claspers", which channel semen into females during mating?
In a new study, researchers at the University of Florida attribute the growth of the two appendages to a gene known as Sonic Hedgehog(obviously named by Sega enthusiasts). They determined that the gene is expressed for a longer period of time in males than in females, thus causing the growth of the claspers later in fin development.
As part of their study, the team investigated the effect of Sonic hedgehog exposure on skate embryos. When given a Sonic hedgehog inhibitor, male skates did not develop the claspers. When the gene's expression was prolonged in female skates, they began to develop claspers, too.
The earliest known vertebrate copulatory organs are claspers, paired penis-like structures that are associated with evolution of internal fertilization and viviparity in Devonian placoderms. Today, only male chondrichthyans possess claspers, which extend from posterior pelvic fins and function as intromittent organs. Here we report that clasper development from pelvic fins of male skates is controlled by hormonal regulation of the Sonic hedgehog (Shh) pathway. We show that Shh signalling is necessary for male clasper development and is sufficient to induce clasper cartilages in females. Androgen receptor (AR) controls the male-specific pattern of Shh in pelvic fins by regulation of Hand2. We identify an androgen response element (ARE) in the Hand2 locus and present biochemical evidence that AR can directly bind the Hand2 ARE. Together, our results suggest that the genetic circuit for appendage development evolved an androgen regulatory input, which prolonged signalling activity and drove clasper skeletogenesis in male fins.
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