Re: Does the number of chromosomes in an organism directly affect complexity?

As might be intuitively expected, there is some evidence suggesting a general trend in the direction of a greater chromosome number for the more complex organisms, exemplified by the single chromosomes of bacteria compared to the 23 pairs in humans.  However, there is definitely not a a direct correlation, upon closer inspection.  Examples of some Numbers of Pairs of Chromosomes in Different Species of Plants and Animals are shown in Table 3-1 in the online textbook, An Introduction to Genetic Analysis from the N.C.B.I. Bookshelf.  A number of exceptions to the trend suggested above are evident, perhaps most notably the 24 chromosome pairs of the potato compared to the 23 pairs of humans.  The disassociation between chromosome number and evolutionary status is reinforced by the text accompanying this Table describing the Topography of the Chromosome Set, which states that "Different species have highly characteristic chromosome numbers... The range is immense, from two in some flowering plants to many hundreds in certain ferns."

As suggested by this Genetics Lecture graph comparing the Variation in Genome Size among Prokaryotes & Eukaryotes, from the Memorial Univ. of Newfoundland, the size of the total genome would appear to correlate better with evolutionary status than chromosome number.  However, this graph illustrates that there is still considerable variation in genome size among organisms in the same class, such as the flowering plants, which skews this relationship.

The genome sequencing & mapping that is currently underway for a wide variety of organisms is producing large amounts of data to enable more accurate evolutionary comparisons, such as this.  The section describing The Comparative Genome Sizes of Humans and Other Organisms Being Studied in the Functional and Comparative Genomics Fact Sheet from the Human Genome Project Information website of the U.S. DOE Office of Science states that "Genome size does not correlate with evolutionary status, nor is the number of genes proportionate with genome size."  The data presented in the accompanying table suggest that "Estimated Gene Number" might correlate better than chromosome number or genome size with evolutionary status.  However, a truly definitive analysis of this relationship will have to wait until the genes have been mapped for a wider range of genomes.  I would expect that there will ultimately be variations from a strict correlation between gene number and evolutionary status, as well.

This type of analysis is of great interest for investigations to determine the genes essential for basic life functions.  The Smallest Genome that Sustains Independent Life found to date is that of Mycoplasma genitalium, which expresses only 517 genes, as reported in Life at its Bare Minimum from the Univ. of North Carolina.

Hopefully this info. will give you a feel for the somewhat tenuous relationships between chromosome number, genome size, gene number, & evolutionary status.  Thanks for the great question,

Jeff Buzby, Ph.D.
MadSci Genetics Network
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