■ Haworthia Study No. 23 (2010. 8) p.6～13 「DNA vs Morphology By Dr. Hayashi」
DNA vs Morphology By Dr. Hayashi
Treutlein & al. (Taxon 52: 193, 2003) reported DNA analysis by matK,
rbcL and ISSR (Fig.1). They indicated that the hard leaf Haworthia group
is closer to Gasteria and Astroloba than to the soft leaf Haworthia and
suggested that the hard leaf Haworthia would be better separated from the
genus Haworthia.
There are many severe defects, however, in their study:-
1. They got most material from botanical gardens in Germany, which are
old collections without locality data. Fig. 3 is a photo of their “H. geraldii”
sent by the authors to me. It is definitely not H. geraldii, but a form
of H. attenuata. I informed them that it was H. attenuata and asked them
to send photos of other materials, but no reply was received. It is obvious
that they used their material without any identification of the names.
2. Aloe aristata and Haworthia aristata have the same DNA accession (registration)
number in their material table. This may be a simple mistake, but rather
indicates rough checking by the Taxon editors.
3. Sequence alignment by the authors is very rough, and they overlooked
many deletions in their registered sequences.
4. They emphasized a high bootstrap ratio in their study, indicating stability of their result. Table 1 details sequence data arranged from their data registered (after re-aliment for deletions), showing common variations among more than 3 materials.
Base pair number supporting a group is counted in Table 1 and added to
Fig. 1 by bold red numbers. For example, group ① (Woody Aloe) is supported
by 7 base pairs (unique changes only) of matK (No. 5~10 in Table 1) and
a rbcL bp (No. 75), and 6 base pairs (matK 1~4 & rbcL 73, 74) are common
with soft leaf Haworthia (Group ③).
For the group ⑤ (Gasteria + hard leaf Haworthia +Astroloba), only 3 base pairs (matK
47, 48, rbcL 82) support the alliance
of this group. As there are no competitive sequences against these 3 pairs, the
group ⑤ will be re-calculated even when only one
of these 3 is taken in a bootstrap sampling.
Bootstrap ratio does not guarantee the validity of the group, especially
for the group supported by a few base pairs. It is rather a mathematic deceit
in such cases. Table 1 may reveal the real validity of the group better than
the bootstrap ratio. DNA researchers must present this kind of table.
Fig. 4 is a cladogram by Ramdhanii & al. (Alsterworthia Int. 9: 13,
2009) using ITS1 and trnL-F. In this figure hard leaf Haworthia species
are also put close to Gasteria and Astroloba, but far from soft leaf Haworthia.
Some other DNA studies (mostly by chloroplast DNA) report similar results.
The results of these DNA studies, however, disagree with traditional taxonomic grouping in Aloaceae, using genetic closeness presumed through hybridization and especially floral structure. It is well known that hard leaf Haworthia hybridizes with Astroloba or Gasteria, but it hybridizes far easier with soft leaf Haworthia.
In morphology, all the Haworthia groups, Astroloba and Chortolirion share
many unique distinctions, especially in their floral structure shown in
Table 2 (cf. Fig. 5). Mark ◎ indicates unique (not found in any other groups
in Aloaceae) distinctions of the group without any exceptions. Mark 〇 indicates
distinctions without exception but not necessary unique. Mark △ shows unique
distinctions with exceptions.
There are many common distinctions among both (soft and hard leaf) Haworthia groups, Astroloba and Chortolirion as shown in pink in Table 2. They share 9 unique floral distinctions without any exceptions (Character No. 1~9), and 6 non-unique floral distinctions without exceptions (Char. No. 11~16). These floral distinctions strongly support alliance among these groups.
It is also shown in Table 2 that 4 floral distinctions (Character 1, 3, 5, 7) clearly separate 3 major groups of Aloaceae (Aloe, Gasteria and Haworthia). It also indicates that Aloe shares several floral distinctions with the Gasteroid (Gasteria and Poellnitzia) and Haworthia is rather isolated from others. These accord well with the genetic closeness estimated by hybridization.
On the other hand, there are some characters which support the alliance
among hard leaf Haworthia, Astroloba and Gasteria as shown in blue in Table
2. This alliance agrees well with the result by DNA analysis to date. There
are, however, only 2 unique distinctions (Char. 28, 29) in these characters
and they are all leaf tissue characters.
Some people may say that DNA data is more important and valid than morphological. It is believed that DNA regulates morphological characters and, hence, it is more reliable than the morphological data, but we should note the problem of DNA data on the following 4 points.
1. Most DNA sequences studied in Aloaceae to date are of chloroplast DNA.
No genes in nuclear DNA have been studied. Of course it is possible to
trace phylogenesis by meaningless (junk) sequences. In such a case, however,
there is no theoretical priority in DNA data over morphological one.
2. It is easily presumed that chloroplast DNA may relate to leaf nature.
Both Treutlein & al. (2003) and Ramdhanii & al. (2009) mostly depend
on the chloroplast DNA data. It is rather natural, therefore, that their
results support the alliance (grouping) by vegetative characters in Table
2 (shown in blue). But it is well known that the grouping by vegetative
character has lower validity than those by floral character.
3. Genome size in Monocotyledons is reported as Rice = 430 million (4.3
x 108) base pair, Sweet corn = 5 billion (5 x 109) bp, Wheat = 17 billion
bp and Fritillaria (Liliaceae) = 120 billion bp. Though genome size is
not known in Aloaceae, if it is 5 billion bp and the average size of a
DNA region is 5000bp, there may be a million DNA regions in Aloe or Haworthia.
Only 5 DNA regions in Aloaceae have been studied to date, 3 of which are
of chloroplast. The others are meaningless regions of nuclear DNA. It is
too immature to say something firm based on such studies by a few and nonsense
DNA regions.
4. The importance of data or character in taxonomy or cladistical analysis
is not decided by itself, even if it is a gene or exon with important function.
It is only evaluated by comparison with other data or characters.
For example, the number of cotyledons is one of the most important characters
to separate dicotyledon and monocotyledon. But it is a vegetative character
and there is no theoretical reason why the number of cotyledons is so important
in taxonomy. It indicates a large grouping/division in Angiospermae together
with many other characters, such as the basic number of perianth, type
and location of vascular bundle, type of leaf veins, existence/lack of
cambium etc. A character supporting a large grouping together with many
other characters is considered important.
On the other hand, the number of stamens, a character used by Linnaeus,
is not used today, though it is a floral character. This character disagrees
with the grouping supported by many other characters. It is considered
not important.
It is the same with DNA data. If results by DNA data disagree with a large grouping by morphological data, the researcher must collect other data (DNA/ morphology) to support his result. In many cases, if the grouping by DNA data is correct, the researcher also found many morphological characters supporting his result.
In DNA analysis, we can get much clear data, and many taxonomic systems have been rearranged by them. As traditional major groups in the Aloaceae have firm floral distinctions, the researcher, who tries to change this grouping, should present a firm base in other floral characters as well as DNA data.
▲ |
