Plants show greater complexity in gene arrangement

Rcjohnsen rcjohnsen at
Mon Feb 14 13:23:47 EST 2000

Chromosomes show plants' secret complexity
O. Baker
Sci News 156:389  Dec 18 & 25 99

   They make life look so simple, but they do it with an enormous number of
genes.  One and a half billion years after plants' lineage diverged from ours,
our green cousins are finally receiving the credit they deserve.
   In a major milestone, nearly 300 researchers working in the United States
and Europe have detailed the exact DNA sequence of two chromosomes from a
plant. In doing so, they've uncovered a surprisingly dense crowd of genes.
   "Plants are a lot more complicated than we give them credit for," comments
Elliot M. Meyerowitz, a molecular geneticist at the California Institute of
Technology in Pasadena.
Nearly 8,000 genes populate chromosomes 2 and 4 of the wild mustard plant
Arabidopsis thaliana, the scientists report. The count dwarfs the recent gene
estimate of about 550 for human chromosome 22 (SN: 12/4/99, p. 356).
Because Arabidopsis has only five chromosomes, it probably doesn't exceed a
person in overall genetic complexity.  However, worms and flies fall short of
matching mustard gene-for-gene, says Meyerowitz.
   "Plants do a lot of things that animals  don't," he adds. For example,
plants tune their development to the circumstances into which they are born.
"You get a very different plant in cold or warm, light or shade—but a fly is a
fly every time," he says.
   Separate teams of scientists tackled chromosomes 2 and 4 of Arabidopsis. 
Both groups report the sequences they elucidated in the Dec. 16 NATURE.
Additional scientific teams scrutinizing the mustard plant's three remaining
chromosomes expect to complete the task within a year.
   The scientists made mustard their target because of the small size of its
chromosomes—implying that they contain relatively little junk DNA interspersed
with the genes. Arabidopsis is also a favorite laboratory organism of
geneticists. The existing base of knowledge will enable scientists to
capitalize quickly on revelations about the Arabidopsis genome.
   Piecing together the interlacing roles of the myriad new genes presents a
massive puzzle, yet many of the pieces are familiar. By comparing the sequences
with those already reported for the mustard and other organisms, the
researchers say that they can guess, at least roughly, the biochemical task
performed by about 60 percent of the Arabidopsis genes.
Even with the detailed role of their genes uncertain, the chromosomes provide
biologists with a snapshot of plant evolution. Many genes have similar
counterparts on the same chromosome, suggesting that Arabidopsis progenitors
duplicated them in recent history. An entire quarter of chromosome 2 shows up a
big chunk of chromosome 4. Biologists suspect that gene duplication has been a
common route to new traits.
   Gaps exist in both chromosome sequences around the knoblike centromeres.
These regions contain repetitious sequences that stymie the technique that
scientists use to break down genome and then assemble its sequence from small
The 16 million bases of the long arm of chromosome 2, however, make up one of
the longest continuous segments of DNA ever sequenced—second only to a piece of
human chromosome 22 unveiled earlier this month.

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