[Picture of <em>Carex aquatillis</em> chromosomes]

Carex Chromosomes (April 2008)

Carex chromosomes are unusual. People have lots of questions about their implications for Carex evolution, but not much is known. (A chromosome is a long DNA molecule packaged in proteins. Each one contains intructions for making and running bodies.) Recently Carex researcher Andrew Hipp wrote three articles (some with Tony Reznicek, sedge guru; Paul Rothrock, chromosome counter extroridinaire; Eric Roalson, who works on Carex phylogeny; and Paul Berry, plant taxonomist) summarizing what is known and adding their own research results. (See bottom of this entry for citations.) Some of the highlights are explained here, with some of my own interpretaion for which Hipp should not be held responsible.

Carex have unusually diverse chromosome numbers. Chromosome numbers are often reported as the number of chromosomes in each set (n). Humans have 46 chromomes in two sets, 23 from the father and a matching set of 23 from the mother. Therefore, humans n = 23 chromosomes. In Carex, chromosome numbers include every number from n = 6 to n = 47, plus several additional numbers up to n = 66. That is very strange. In more than 100 species, individual plants are known to have have different numbers of chromosomes in each set, up to ten different chromosome numbers in a species. That is also very strange.

In most plants, a high number of chromosomes is the result of polyploidy, the addition of whole sets of chromosomes. For example, a Red Fescue grass with 42 chromosomes has 6 sets of chromosomes (three from each parent); n = 7, 6n = 42. However, most Carex with high chromosome number are probably not polypoid. That is strange.

Why are Carex chromosome numbers so strange?

During cell division, the movement of chromosomes is controlled by fibers that attach to a chromosome at a point called the centromere. In most organisms, each chromosome has just one centromere. As a result, if a piece of chromosome breaks off, it is lost.

In Carex and some other species, the chromosomes have “diffuse centromeric activity;” there are lots and lots of points where the fibers attach to each chromosome. Such chromosomes are called “holocentric.” If a holocentric chromosome breaks, the pieces move normally during cell division.

Holocentric chromosomes also occur in woodrushes (Luzula), parasitic dodders (Cuscuta), sundews (Drosera), nutmeg (Myristica fragrans) and a lily-like Asian plant (Chionographis), as well as nemotodes (roundworms) and several groups of insects including bees, some moths, a group of true bugs, and dragonflies.

High chromosome numbers in Carex can result from chromosome breakage. A plant with n = 35 might have the same amount of DNA as one with n = 6, but broken in smaller pieces. The small pieces weren’t lost because the chromosomes are holocentric.

Biologists have wondered if Carex are ever polyploid. There is now evidence that three Carex species are probably polyploids. No doubt some others are too. However, it looks like most Carex species have just two sets of chromosomes, even if they have many chromosomes. (This should be checked with more research.)

Does this matter? Maybe yes. Maybe the pattern of chromosome breakage (or fusion) drives the evolution of new Carex species.

One early hypothesis was that ancestral Carex had few, large chromosomes and the new species evolve as chromosomes break, so later species always have larger numbers. That idea may be probably partly true. It does seem likely that ancestral Carex did have n = 6 (or 5 or 7) and the chromosome number mostly got bigger (by breakage) because it couldn’t very well get smaller. However, the idea that sedge chromosome numbers only get bigger (as chromosomes break into small pieces) is too simple. Consider the Ovales sedges. Their ancestor probably had n = 35 (approximately). Some more recently evolved Ovales have diverse chromosome numbers, from n = 20 to n = 40 or more. Apparently both chromosome breakage and chromosome fusion occurred during Ovales evolution. This is mildly interesting.

However, holocentric chromosomes may drive Carex evolution in a another, more important way. If chromosomes repeatedly beak and fuse in one family line, they reach a point where they don’t match up very well with chromosomes from a different family line, during the kind of cell division that produces pollen and eggs. Take the example of Carex pachystachya. It may have n = 37, 38, 39, 40, or 41. If parent C. pachystachya with different chromosome numbers cross, the offspring survive very well. However, they tend to be infertile because their chromosomes don’t line up in the proper pairs during formation of pollen and eggs. It looks like chromosome breakage (and fusion) in Carex can produce populations that are reproductively isolated from each other ­ even if look very similar and live in similar habitats. And we all know that the genus Carex is full of distinct species that look similar!

All this suggests that the driving forces of Carex evolution may be the opposite of what I had thought. Many Carex species are microhabitat specialists ­ a species may live only in a very precise, limited habitat. I thought Carex are diverse because they can split the world up into many different ecological niches, so many Carex species can fit in the world. It looks like the number of Carex species may be high because of chromosome breakage (and fusion), resulting from the behavior of holocentric chromosomes. Therefore, many Carex species form and they can survive only if specialize in narrow ecological niches, minimizing the competition that would otherwise drive many of them extinct. (A lot more research is needed before we really know whether chromosomes or ecology can be considered a primary “cause” of Carex diversity.)

All this brings up other interesting questions. For one thing, if holocentric chromosomes in some way cause Carex diversity, why aren’t the other sedge genera more diverse? (They have holocentric chromosomes too.) And if holocentric chromosomes somehow discourage polyploidy, why is polyploidy moderately common in other sedge genera? How are DNA sequences different in holocentric chromosomes compared to typical chromosomes that have a single centromere? How do those differences affect gene expression? Are some rearrangements favored and others rejected by natural selection because of “diffuse centromeric activity,” and if so, what rearrangements and why?

(And for a real biology nerd, here are a couple even more technical ways Carex chromosomes are odd: The chromosome number is reduced in the first cell division of meiosis, not the second as in most organisms. Also, when one pollen mother cell divides, three of the resulting cells degenerate and only one pollen grain is produced; in most plants, each pollen mother cell produces four pollen grains. How do these differences affect Carex evolution, and how do they relate to holocentric chromosomes?)

Hipp, Andrew L. 2007. Nonuniform processes of chromosome evolution in sedges (Carex: Cyperaceae). Evolution 61: 2175-2194.

Hipp, Andrew L., Paul E. Rothrock, Anton A. Reznicek, and Paul E. Berry. 2007. Chromosome number changes associated with speciation in sedges: a phylogenetic study in Carex section Ovales (Cyperaceae) using AFLP data. Aliso 23: 193-203.

Hipp, Andrew L., Paul E. Rothrock, and Eric H. Roalson. In press. The evolution of chromosome arrangements in Carex (Cyperaceae). The Botanical Review.