Re: What is chromosome number (2N[equ]?) of whitefish (used for animal mitosis)?

Whitefish are members of the Salmonidae, in the genus Coregonus. You will probably have seen pictures of whitefish mitosis in many different textbooks, since that organism was a readily available animal in the early history of cytogenetics, and has excellent tissues for observing mitosis. You have also probably been frustrated in not being able to find the diploid (2N) number for whitefish listed in any of these photomicrographs. Of course you have, that is why you are asking the question!

Well, the answer is quite simply that there is no unique species or population of whitefish used to provide those photomicrographs. In fact, the population cytogenetics of fish species is very difficult, because of their tendency towards hybridization and autopolyploidization. (Note: autopolyploidization is the occurrence of doublings of chromosome number by failure of chromosomes to divide equationally in a mitosis following chromosome replication.) Like plants, fish seem to have commonly used polyploidization as an evolutionary tool. This may be allowable because many fish, like plants, lack specific sex chromosomes for sex determination. A little thought about X and Y chromosomes for sex determination and polyploidization will probably tell you why these things are generally evolutionarily incompatible.

These complications in whitefish evolution blur the distinction between one population of these fish and another; it even blurs the distinction between true species members of the genus.

The best source for information on fish cytogenetics in general, is a little book called Fish Chromosome Methodology, by Thomas E. Denton, published by Charles E. Thomas, Springfield, Illinois. In a table in this book, there are 19 species of Whitefish listed, varying in chromosome number from 2N = 36 to 2N = 96, with some species even listed with different chromosome numbers among races! I think it is a safe bet that MOST textbook pictures will have used fish with the smaller number of chromosomes, for maximum visibility, but this is by no means certain. Thus, there is no real answer to your question, I am afraid.

I am sorry to say that I really don't know of any good source for chromosome numbers of various species, either on the net or in a summary book. Most introductory genetics texts have at least one table which presents the chromosome number of a score or more of species, but the species are rather haphazardly chosen, and may differ considerably from text to text, depending on the author's organismal bias. Usually, surveys of chromosome numbers are found piecemeal, for certain groups of organisms, in specialty texts on those organisms. If you find any such table in cyberspace, I would certainly appreciate knowing about it!