22kb inversion in sunflowers

R. Joel Duff (Virkotto@intrnet.net)
Fri, 20 Feb 1998 15:21:13 -0600 (CST)


Just to follow up my own post with another example I thought I would talk
about the sunflower family. The Asteraceae is one of the largest
(Orchidaceae being the largest I believe) families of flowering plants. It
consists of some 1100 genera and 20,000 species. These are everything from
common sunflowers, goldenrod, to daisies and Dahlias. There are shrub,
tree, annual, and perenial species that grow in nearly every environment on

The chloroplast genome of plants is a circular genome about 160,000 bps in
length and contains about 123 genes. The order of these genes is almost
identical among all land plants. Several groups of plants are
characterized by differences in this "consensus" gene order typified by
Tobacco. Some examples: All bryophytes (mosses, liverworts, and
hornworts) and lycopods (Lycopodium, Isoetes, and Selaginella) have a 30 kb
(1000bp) inversion relative to "higher" plants. Rice has a 78 kb inversion
(overlapping the position of the inversion in "lower" plants). All legumes
(beans, alfala, redbud, etc..) have a 50 kb inversion relative to tobacco.
The one I want to focus on is the inversion in the Asteraceae.

The 1100 genera of the Asteraceae are split up into 13 tribes and those
into subtribes. Several specialized morphological features strongly
support the naturalness of this family. Of all of these tribes, members of
12 of these tribes all contain a 22 kb intron in their cpDNA genome. Have
ALL been examined, no but representative of most of the genera have been
scanned and thus far everyone has this inversion. Only the genera of the
Tribe Barnedesiinae lack the inversion (have the consensus plant gene
order). What is interesting about this is that it had for some time been
concluded based on morphological, DNA sequence and RFLP data that this
tribe represented the basal lineage of this family. The most parsimonious
evolutionary explanation is that that from the split of the ancestors of
the Barnedesiinae to the other tribes an inversion occurred in which one
species ended up with a rearranged genome. I have not heard anyone suggest
that the direction in which the genes are oriented has anything to do with
the expression of these genes which suggest there is little competitive
advantage or anything about the gene order that would suggest that it makes
a bit of difference to the members of the 12 tribes that have this
rearrangement. I do not say that God could not have created the plants
with whatever gene order he wanted or that there couldn't be a common
design involved but why should this gene order be present in a group of
organisms all suggested to be related by independent anlayses?

Within the 78 kb inversion in all legumes there are several smaller
inversions that are specific to either species or groups of genera. In
this case one might have 10 species (for example one that occurs in the
common bean tribe) that are very similar each to one another but 5 will
have a small inversion while the others do not. Are these all created this
way or were there 5 independent inversions all occurring at the same
position in the nucleotide sequence (this can hardly be due to functional
or environmental influences)?

Beyond the challenge to 6-day creationists, I think some people in the ID
camp would have little problem with the examples I have given but what of
PJ? How would the latter deal with the appearance of history in organisms
he might accept as being bound by a common ancestor. I suppose in the bean
example he could except this as some type of microevoltion but if one takes
that approach how does one deal with identical data when looking at broader



Joel and Dawn Duff / | ' \ Spell Check?
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