Tions and ligers and Cher, oh my!

this question is about freak animals. You know the ones I’m talking Bout, chimeras and intraspecies loving. Why does mating two species result in a sterile animal?

Is it a hormonal thing, or are the animals missing the source cells necessary for sperm and egg production, or are they completely internal-genitalialess?

Basically, what is the mechanism that allows the reproduction, but thwarts the next cycle of reproduction?

fanks,
jb

Um… something to do with a croma… err cromoz… err croizone number different between the two species?
That is a WAG but I felt the need to bump this off the bottom of page two.

Osip

OK, Sonny, until a biologist comes along:

Mules are fully equipped, and functional in the mechanics of procreation. In fact, it is reccomended that male mules be castrated since they tend to be randy beasts otherwise. Mules produce sperms and eggs, which just don’t work (generally - there are apparently occasional instances of fertile female mules).

In many hybrids, such as mules, the parents have differing numbers of chromosomes. While this still allows the production of offspring in closely related species, the chromosomes are unevenly matched, and it makes meiosis leading to viable gametes nigh unto impossible - taking a random “half” of chromosomes from the hybrid individual is bound to have some instructions missing somewhere.

how is it possible for differently chromosomed parents to combine their genetic material successfully? That’s what I don’t get. is there an established limit to the numerical chromosome difference allowed for two species to mate? or is there no fixed rule, and zygote production depends on the species by species basis of the putative parents?
and on a rlatd question, I remember reading as a lad (maybe in 1,000 Things Everyone Should Know About Science, now Sharks Have No Bones) that a particular species of ant only has one chromosome. How does that work? (meiosis, mitosis, sexual reproduction, etc)

thanks,
jb

To use equines again, most equine species can hybridize. In particular, zebras can be crossed either with horses or donkeys to produce a hybrid “zebrule” (some sources will tell you this is only a term for a specific cross). One species of zebra has 44 chromosomes to the horses 64, but they can still hybridize.

The chromosome count apparently doesn’t much matter for the genes to cooperate in building an organism, as long as the instructions aren’t radically different, ie. if they are closely related species. Packaging into chromosomes would seem to be chiefly a mechanism to allow meiosis to take place, and here, the hybrid is out of luck - it has some intermediate number of chromosomes (mules have 63, intermediate between the horse’s 64 and the donkey’s 62), many of which are unpaired, meaning that the process of meiosis produces a gamete with a lot of missing information.

I’m using the mules because I dug up some stuff on the nature of mules for another thread sometime back, and because they’re probably the most familiar case of hybridization in mammals:

http://boards.straightdope.com/sdmb/showthread.php?threadid=35128

For any “deeper” explanation on the mechanisms involved, I’m out of MY depth. Wait for a biologist.