Male or female? First sex-determining genes appeared in mammals some 180 million years ago

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The Y chromosome, which distinguishes males from females at the genetic level, appeared some 180 million years ago. It originated twice independently in all mammals. Scientists have managed to date these events that are crucial for both mammalian evolution and our lives, because the Y chromosome determines whether we are born as a boy or girl.

Man or woman? Male or female? In humans and other mammals, the difference between sexes depends on one single element of the genome: the Y chromosome. It is present only in males, where the two sexual chromosomes are X and Y, whereas women have two X chromosomes. Thus, the Y is ultimately responsible for all the morphological and physiological differences between males and females.

But this has not always been the case. A very long time ago, the X and Y were identical, until the Y started to differentiate from the X in males. It then progressively shrank to such an extent that, nowadays, it only contains about 20 genes (the X carries more than one thousand genes). When did the Y originate and which genes have been kept? The answer has just been brought to light by the team of Henrik Kaessmann, Associate Professor at the CIG (UNIL) and group leader at the SIB Swiss Institute of Bioinformatics, and their collaborators in Australia. They have established that the first " sex genes " appeared concomitantly in mammals around 180 million years ago.

4,3 billion genetic sequences

By studying samples from several male tissues — in particular testicles — from different species, the researchers recovered the Y chromosome genes from the three major mammalian lineages: placentals (which include humans, apes, rodents and elephants), marsupials (such as opossums and kangaroos) and monotremes (egg-laying mammals, such as the platypus and the echidna, a kind of Australian porcupine). In total, the researchers worked with samples from 15 different mammals, representing these three lineages, as well as the chicken, which they included for comparison.

Instead of sequencing all Y chromosomes, which would have been a " colossal task " according to Diego Cortez, researcher at CIG and SIB and main author of the study, the scientists " opted for a shortcut ." By comparing genetic sequences from male and female tissues, they eliminated all sequences common to both sexes in order to keep only those sequences corresponding to the Y chromosome. By doing so, they established the largest gene atlas of this " male " chromosome to date.

Written By: Science Daily
continue to source article at sciencedaily.com

4 COMMENTS

    • In reply to #1 by Red Dog:

      I always assumed that sexuality was just a trait common to virtually all animals and occurred in a common ancestor well before mammals. This is fascinating.

      Red Dog, your assumption is still accurate. This is not study on the evolution of sexuality, but on the mechanisms of sex determination in our lineage. The end of the article makes clear that a different form of sex determination existed before the evolution of our present genetic mechanism. and males and females were determined by a different mechanism, possibly temperature. It’s just by chance that we have the system we do. Birds have a similar one but it’s switched with the female as the XY. Obviously the origin of sexuality and the two sexes itself goes back WAY longer than a measly 180 million years, this should be obvious.

      • In reply to #2 by Brianna Socks:

        In reply to #1 by Red Dog:

        Red Dog, your assumption is still accurate. This is not study on the evolution of sexuality, but on the mechanisms…

        Thanks for that. Now I get it, I thought that sexuality went back further. This is why I like this site, I can learn as much from the comments as the articles.

    • In reply to #1 by Red Dog:

      I always assumed that sexuality was just a trait common to virtually all animals and occurred in a common ancestor well before mammals. This is fascinating.

      Sexuality goes way back to hermaphrodite organisms which have both sexes in the same individual. – With hermaphrodite organisms such a snails still exhibiting that condition. http://animals.pawnation.com/list-hermaphrodite-animals-2829.html

      Then there are sequential hermaphrodite fish, which change from one sex to the other, at some stage in their lives.

      Lessons from the pond: Clues from green algae on the origin of males and females – http://www.sciencedaily.com/releases/2010/04/100415141125.htm

      A multicellular green alga, Volvox carteri, may have finally unlocked the secrets behind the evolution of different sexes. Scientists have shown that the genetic region that determines sex in Volvox has changed dramatically relative to that of the closely related unicellular alga Chlamydomonas reinhardtii.

      Then, as Brianna Socks points out some vertebrates such as crocodiles have sex determined by the incubation temperature of the eggs in their nests.

      http://en.wikipedia.org/wiki/Crocodile#Reproduction

      Sex is determined by temperature, where at 30 °C (86 °F) or less most hatchlings are females and at 31 °C (88 °F), offspring are of both sexes. A temperature of 32 to 33 °C (90 to 91 °F) gives mostly males whereas above 33 °C (91 °F) in some species continues to give males but in other species resulting in females, which are sometimes called as high-temperature females.[65] Temperature also affects growth and survival rate of the young, which may explain the sexual dimorphism in crocodiles.

      This makes these species vulnerable to global temperature changes.

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