About K Shows Expertise I am able to answer questions regarding human medical genetics, mouse genetic studies, molecular biology, biochemistry, and cell biology. I am also able to answer questions concerning prenatal and postnatal genetic testing but am ethically unable to give advice on such test results.
Experience I have 10 years experience in human genetics, including clinical, population, and molecular genetics. I am currently an active researcher in molecular and developmental genetics at Virginia Commonwealth University/ Medical College of Virginia. I have previously taught genetics and cell biology at the college level.
Organizations American Association for the Advancement of Science (AAAS)
American Society of Human Genetics
Education/Credentials B.S., Biochemistry, University of Southern Mississippi
M.S., Human Genetics, Louisiana State University Medical Center
Ph.D., Genetics and Vision Science, University of Alabama at Birmingham
Question there is speculation over whether all hybrids are sterile, i am doing an essay on ligers and i know the females arent sterile but the males are. i was wondering if you could tell me why they are sterile in terms of genetics.
Answer Leah,
I do not know what causes sterility in females but not males. I was under the impression that these creatures were urban legends, however, I looked them up and, lo and behold, there were tons of photographs of them. CAVEAT: However, any information you find on the internet, unless it is research conducted by a zoo or scientist, can not be used as factual. No scientific study has been performed yet on lion/tiger hybrids.
What usually causes sterility in mammal hybrid crosses is that the chromosomes do not pair evenly during meiosis, or gamete formation. There is a stage during meiosis that has to occur called homolog pairing and crossing over, which can not occur if chromosomes are not the same size or shape. During the first stage of meiosis the two sets of chromosomes double so that there are four copies of each chromosome. The four similar chromosomes (called chromatids at this point) all align in a homolog pair. The actual physical exchange of genetic material then occurs, at least once per chromosome. If this does not occur, then the cell does not divide and the sperm or egg is sterile. If this always happens because the chromosomes do not align, then the animal is sterile. In addition, if the chromosomes each contain the same genetic material, but it is rearranged, then unequal crossing over can occur and result in duplications or deletions of parts of chromosomes-- if a critical gene is deleted or duplicated the animal may not survive to term or even to adulthood. This is evidenced by the great number of stillborn and neonatal lethal ligers that are born each year. In addition, some pairings are more successful than others, such as bengal tigers with african lions, which suggests that their chromosomes may be more compatible than other combinations. (It may also have to do with size-- think of a female chihuahua and a male great dane mating.)
You have pointed out that male ligers (a cross only between a female tiger and a male lion; the converse cross between a female lion and a male tiger is less common) are sterile, while females are fertile. In males there is only one X, so it pairs with the Y in a small region. I would suspect that the difference between a tiger X and a lion X is pretty small, but the difference between a tiger Y and a lion Y is pretty big. This is probably why a female liger with an X of each type is ok, because she can go through proper crossing over.
Now, why are ligers viable in the first place? They are all members of the Panthera genus, so it is possible that the chromosomes are enough conserved evolutionarily so that they can breed. Like domestic dogs, the big cats may represent more of different breeds than different species. The big cats split very recently in evolution, so they have not evolved apart as much as, say chimps and humans. So they probably share quite a bit of evolutionary conservation among their chromosomes so they can get lucky and produce a viable offspring. I am not aware of any chromosomal studies of the big cats besides number (lions and tigers both have 38, or 19 pairs). The lion and tiger may then be subspecies of each other because they can mate and sometimes produce viable offspring.
The most likely explanantion for sterility, however, is the unequal crossing over during meiosis that causes deletions and duplications of genetic material.
