I'm hoping for some expert opinions and help here.
Over the course of evolutionary history, life on this planet most likely had to move from a form that had only a single, unpaired chromosome to the vast variety we see today.
I'm curious as to the different causes and issues involved with this progression.
For asexual creatures, it's really not that big of an issue (or is it?) seeing as how they don't have to find mates. Sexual lifeforms however have a must more difficult time modifying their chromosome counts.
I found a great article discussing Robertsonian fusions which can reduce the number of chromosomes in a population. It was (according to the article) the method by which humans moved to 23 pairs from the 24 pair ape population.
What other methods are out there that change chromosome counts? How do the new creatures procreate? Is it through inbreeding with siblings with the same 'disorder'? If so, that is the ultimate population bottleneck encompassing a single family.
quote:Now differences in chromosome number do not serve as reproductive barriers between all species. For example, lets look at some of the equine species ( horses and donkeys). Domesticated horses have 32 pairs of chromosomes and Donkeys have 31. Yet, they can produce offspring, mules, which have 31.5 pairs of chromosomes. One of the horse chromosomes goes unpaired. Wild mountain zebras have 16 pairs of chromosomes, while the last species of wild horse (Przewalski's Horse) has 33 pairs. However, all of these equine species can produce hybrid offspring. In all of these crosses but one, the offspring are sterile. It has long been argued that this sterility is due to the difference in chromosome number, but hybrids of the wild (33 pairs) and domesticated horse (32 pairs) are fertile, and have 32.5 pairs of chromosomes. So clearly, something more than just differences in chromosome number is contributing to the species interbreeding barrier.
quote:A donkey has 62 chromosomes; the zebra has between 44 and 62 (depending on species). In spite of this difference, viable hybrids are possible provided the gene combination in the hybrid allows for embryonic development to birth. A hybrid has a number of chromosomes somewhere in between. The chromosome difference makes female hybrids poorly fertile and male hybrids sterile due to a phenomenon called Haldane's Rule. The difference in chromosome number is most likely due to horses having 2 longer chromosomes that contain similar gene content that contain the same genes as 4 zebra chromosomes.
So are there hybrids between the different zebra species? Are they fertile?
Seems to me that when there are fused chromosomes in some species closely related to others without the fusion, that they could - occasionally - have both of the non-fused versions match up to the one fused one in hybrids.
quote:Since fertility barriers likely isolated this tetraploid lineage from its ancestors, instantaneous speciation, although rare is possible in mammals, and a role for doubling series variation in genome size to trigger evolutionary novelties is suggested by this unique tetraploid rodent. Funded by Fondecyt 1970710 and the Fulbright Commission.
It certainly poses a problem for reproduction to have some paired chromosomes and some unpaired ones, as demonstrated by the numerous cases of infertile hybrids between species with only 1 or 2 differences in numbers.
Would it be possible for a tetraploid egg to be fertilized by two diploid sperm? A diploid egg to be fertilized by one tetraploid sperm and one diploid sperm? Both could generate matched pairs of chromosomes in the zygote that would now be also tetraploid without necessitating sibling mating.