Sci­en­tists get peek into how some fish change sex

The Prince George Citizen - - Science - Jeremy REHM

NEW YORK — If in the be­gin­ning there was male and fe­male, fish seem to have for­got­ten the memo.

For nearly 500 fish species, in­clud­ing the clown­fish in Find­ing Nemo, the great di­vide be­tween sexes is more like a murky line: if cir­cum­stances call for it, the fish can swap their sex, with fe­males turn­ing into males in some species and males turn­ing into fe­males in oth­ers.

Peo­ple think of sex as be­ing fixed, said bi­ol­o­gist Erica Todd from the Univer­sity of Otago in New Zealand, “but there are so many fish that can push it in the other di­rec­tion.”

Sci­en­tists have known for decades about the sex trades, but they’ve had lim­ited un­der­stand­ing of how the ex­change hap­pens.

In a study pub­lished Wed­nes­day in Science Ad­vances, Todd and her col­leagues de­tail the molec­u­lar events be­hind this abil­ity, as well as what keeps mam­mals stuck as one sex or another.

The re­searchers looked at the blue­head wrasse, a reef fish that swims in small groups of a dom­i­nant blue-headed male and a posse of smaller yel­low fe­males.

Nor­mally the male and fe­males stay as they are, feed­ing to­gether and oc­ca­sion­ally mat­ing. But if a preda­tor hap­pens to snatch up the lead male, the dom­i­nant fe­male in the group will be­come a male.

“The sex change in this species is re­mark­able be­cause it’s so quick,” Todd said. It takes only min­utes to a few hours for the fe­male’s be­hav­iour to be­come more ter­ri­to­rial and ag­gres­sive like a male. In a few days, she courts other fe­males. And after eight to 10 days, she’s fully tran­si­tioned to a male.

Todd and her team re­moved the lead males from sev­eral wrasse groups in the Florida Keys. As the fe­males changed sexes, the re­searchers took DNA from cells in the an­i­mal’s brains and gen­i­tals so they could fol­low what was hap­pen­ing at the ge­netic level.

They found that re­mov­ing the males likely stressed fe­males. The hor­mones re­leased from that stress dial back the ac­tiv­ity of the gene that makes the fe­male hor­mone es­tro­gen, and even­tu­ally ovary cells start to die.

At the same time, those hor­mones in­crease ac­tiv­ity in the genes that pro­duce male hor­mones, and later tes­ti­cles form.

At a cer­tain point, the re­pro­duc­tive gland “is mostly dy­ing fe­male cells and pro­lif­er­a­tion of early male cells,” Todd said.

But hor­mones weren’t the only thing switch­ing around. The sci­en­tists also saw a com­plete re­arrange­ment of chem­i­cal tags that at­tach to DNA. Th­ese tags turn genes on or off and have spe­cific ar­range­ments in males and fe­males.

As fe­male wrasse tran­si­tioned to a male, th­ese tags were re­moved and re­or­ga­nized, al­most as if the fish was be­ing re­pro­grammed.

“They’re sort of poised and ready to go ei­ther di­rec­tion” like a see­saw, she said. The hor­mones help push it to the male side.

Laura Casas, a bi­ol­o­gist in Spain who was not in­volved in the study, called the re­sults sur­pris­ing. She ex­pects the find­ings can ap­ply to other sex-chang­ing fish, in­clud­ing her study an­i­mal clown­fish, which shift from male to fe­male.

Matthew Grober, of Ge­or­gia State Univer­sity, was more skep­ti­cal, es­pe­cially of stress as the source that trig­gers the change.

He ques­tioned how the fish avoid chang­ing sex from day-to-day stress and sus­pects some­thing else is at play.

All an­i­mals with a back­bone, in­clud­ing hu­mans, share th­ese genes, rais­ing the ques­tion of whether all of them have a deep-seated ca­pa­bil­ity to switch sex.

That’s un­likely. Our re­pro­duc­tive sys­tems are more com­plex and would be far more com­pli­cated to rewire, Todd said.

There’s also cell ma­chin­ery that ag­gres­sively op­poses the see­saw from swing­ing the other way.

“Th­ese fish are just able to go back and re­move that sup­pres­sion,” she said.


A dom­i­nant male blue­head wrasse, up­per left, de­fends its spawn­ing ter­ri­tory and a group of yel­low fe­males off the coast of Florida. If the male dies, the dom­i­nant fe­male in the group will be­come a male.

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