Re: What organism(s) has the most chromosomes? Are they the most complex?

I'm not 100% positive, but I believe the record is held by Indian fern, Ophioglossum reticulatum, with 1260 chromosomes. For the low end, a species of ant, Myrmecia pilosula has only 2 chromosomes as does a type of roundworm Ascaris sp. So chromosome # doesn't actually correlate with the complexity of an organism. (Though I'm sure you can find people who would argue that a fern is just as complex as an ant and vice versa.) It's really more the number of genes expressed. The fern and the roundworm, probably has much less than the 100,000 estimated genes of humans.

Why the excess? Most genomes have a lot of repetitive DNA (for more, see Concept 30, DNA from the Beginning), which may have structural roles in chromosomes. Extra DNA is mostly regarded as genetic filler, evolutionary accumulation of gene junk. Over evolutionary time, DNA packages (chromosomes) could have duplicated or been broken up. These could lead to new species and different chromosome numbers. As long as the genes on the chromosomes are preserved, chromosome # doesn't seem to matter.

Regards,
Shirley Chan
DNA Learning Center