Agriculture Reference
In-Depth Information
and mitochondrial DNA between different and divergent species to occur and
accumulate. Many of these differences have no apparent effect on the form or
functioning of the plant and are therefore not subject to evolutionary selection.
This pool of information can be used to reconstruct the course of evolutionary
divergence without using morphological or anatomical features. In the case of
DNA that is expressed in protein synthesis, these changes may be reflected by
accumulating differences in the proteins - for example, enzymes between
different plants and groups of plants. A number of molecular separation
techniques allow these differences to be detected. Such differences for a specific
gene or DNA fragment are termed polymorphisms, and forms of the same
enzyme with different protein structures, which can be separated by molecular
fractionation techniques, are called isozymes. The extent of difference between
such polymorphic DNA fragments and isozymes between plants can be
quantized and used in measures of difference or divergence between
individuals, species and groups of species. Natural selection for a particular
ecological situation can sometimes drive unrelated species to a similar
morphology or anatomy; this is convergent evolution and can be misleading to
those using such observable (phenotypic) features to classify plants and
reconstruct their evolutionary path (phylogeny). This problem does not arise
when measuring molecular differences that have no phenotypic consequences
for the plants being compared.
The Friesen
et al.
(2006) classification is based on differences in the nucleic
acid base sequence from the intergenic transcribed spacer (ITS) region of the
ribosome DNA from the nucleus of 195 allium species and five species from closely
related genera. Their analysis breaks the whole genus
Allium
into 15 subgenera,
which subdivide further into 63 sections. The subgenus
Cepa
, which includes
onion, is the most divergent or 'advanced' from the related genera outside the
alliums. The subgenus
Cepa
along with subgenus
Allium
, which includes garlic
and leek, are within a distinct 'advanced' group of common ancestry (clade) that
diverges from another group, which includes the subgenus
Melanocrommyum
,
some of which are used as ornamentals (Kamenetsky and Fritsch, 2002).
All the above subgenera fall within a larger grouping that is clearly
separate from a clade which includes the subgenus
Amerallium
, in which are
the North American species with a basic chromosome number of seven. The
molecular data show very high 'genetic distances' within the genus
Allium
.
These distances are typical of those found between subfamilies or even families
in other plants. This suggests that the genus
Allium
is of ancient origin but that
molecular evolution has not been accompanied by a great divergence in plant
type. The molecular differences, together with the distribution area of alliums,
indicate that the genus originated early in the Tertiary geological era, i.e. about
60 million years ago (Friesen
et al.
, 2006).
Rhizomes used to be considered a primitive or ancestral feature in allium
classification and the former subgenus
Rhizirideum
, delineated by this feature,
included
A. cepa
(onions) and
A. schoenoprasum
(chives). Phylogenetic trees
