Annals of Botany 2009 103(8):v; doi:10.1093/aob/mcp125
© The Author 2009. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
John Bryant takes a closer look at some of this month's Original Articles
J. A. Bryant, Professor
University of Exeter, UK
E-mail j.a.bryant{at}exeter.ac.uk
Genomic doubling generates genetic diversity
Polyploidization
(often following hybridization) has played a major role in the
evolution of plants.
Anssour et al. (Jena and Gaterslaben, Germany, pp. 1207–1217) cite earlier papers stating that up to 70 % of angiosperms may
be auto- or allopolyploids. The authors state that polyploidy
leads, in early generations at least, to ‘genetic turmoil’
with the likelihood of new genetic variation. In investigating
some of the genetic consequences of polyploidization they created
the allotetraploid
Nicotiana x obtusiata (
N. attenuata x N. obtusifolia) and autotetraploids of both species. For comparison,
they also looked at natural allotetraploids,
N. quadrivalvis and
N. clevelandii, which arose about one million years ago
from
N. attenuata x N. obtusifolia crosses. We focus here on
a small part of their detailed and thorough study. Firstly,
the genome size of the tetraploid may be less than expected.
Autotetraploids of
N. attenuata and
N. obtusifolia had genome
sizes of only 90 % of the size expected from doubling the diploid
genome sizes. This did not occur in the allotetraploids, although
during the course of evolution
N. quadrivalvis has undergone
a slight increase in genome size. At the molecular level, universally
primed PCR (UP-PCR, a form of DNA fingerprinting) showed that
five different
N. x obtusiata lines had similar but not identical
profiles and that DNA markers from
N. attenuata (70 %) predominated
over those from
N. obtusifolia (28·5 %). After five generations,
1·5 % of the DNA markers represented new variation. The
generation of new genetic variation is seen much more strongly
in the natural allopolyploids,
N. quadrivalvis and
N. clevelandii,
in which new markers account for between 40 and 50 % of the
UP-PCR profile and the excess of
N. attenuata over
N. obtusifolia markers is much reduced. These genomic data, combined with the
accompanying phenotypic descriptions, show us that it is too
simplistic to think of polyploidization simply as the addition
of genomes. There is much more to it than that.
Slippery when wet
Many people, botanists
and non-botanists alike, are fascinated by insectivorous plants.
There is even a video of the growth of a Venus fly trap (
Dionaea muscipula) on YouTube. Amongst the most spectacular insectivorous
species are the pitcher plants in the genus
Nepenthes. Pitchers
take several weeks to develop before finally opening for action.
It has been observed that even after opening, pitchers change
in shape and colour and this has led
Bauer et al. (Cambridge, UK and Wurzburg, Germany, pp. 1219–1226) to ask whether insect-trapping efficiency also changes during
the life of the pitcher. Pitchers of
N. rafflesiana were monitored
during the first 2 weeks after opening (approx. 25 % of an average
pitcher life span). Natural prey capture was very variable but
showed an upward trend over the 2 weeks. This upward trend was
much clearer in trapping-efficiency experiments in which ants
were introduced to the pitchers. The latter data suggested that
the pitchers became more slippery over the 2 week period and
this was confirmed by conductance measurements across the peristome.
Conductance showed a clear diurnal fluctuation but both the
minimum and maximum values increased during the first 8 days,
after which they levelled out or decreased very slightly. These
data show that the peristome surface became wetter over the
first 8 days of pitcher life. Alongside this change, nectar
production increased approx. 10-fold between days 4 and 7 whilst
fragrance, initially almost undetectable, continued to increase
during the 2 weeks of observation. Overall, then, the data indicate
that the pitchers become more efficient at attracting and trapping
prey during the first 2 weeks of life. It is thus slightly surprising
that the viscosity of the digestive fluid decreased significantly
over the same period, as did its pH. This indicated that, notwithstanding
earlier suggestions, the properties of the fluid do not play
a major role in prey capture in this species.
Meadows become battlefields in chemical warfare
When it was first
suggested that competition between plants may involve ‘chemical
warfare’, the idea was met with some scepticism. However,
it is now clear that some plants do release allelopathic chemicals,
although it is often difficult to separate the effects of these
chemicals from other aspects of competition. It is this problem
that has been addressed by
Viard-Crétat et al. (Grenoble, Montpellier and Le Bourget du Lac, pp. 1271–1278) in the grass
Festuca paniculata. This is a dominant species
in sub-Alpine meadows and becomes ‘over-dominant’
if the meadows are not mown, leading to loss of other vegetation
and hence a reduction in biodiversity. In order to test whether
allelopathy is involved, the authors set up three series of
pots containing, respectively, bare soil, unmown
F. paniculata and mown
F. paniculata. These ‘donor pots’ were
watered from above and the leachates from the bottom of the
pots were distributed to target pots containing
F. paniculata,
or
Dactylis glomerata or
Bromus erectus. This arrangement enabled
the authors to study chemical interactions without the complicating
factor of resource competition. Leachates from
F. paniculata significantly inhibited the growth of
D. glomerata and
B. erectus but not of
F. paniculata itself. The plants affected by leachate
also showed a change in biomass distribution as seen in higher
leaf:root ratios. These effects on
D. glomerata and
B. erectus were ascribed to polyphenols, which were present at much higher
concentrations in leachates from
F. paniculata than from bare
soil. Interestingly, the leachates from mown
F. paniculata were
slightly more inhibitory than those from unmown plants, although
this difference was not statistically significant. However,
leachates from mown plants did not contain a higher concentration
of polyphenols. Overall, these data provide evidence for an
allelopathic effect of
F. paniculata on its competitors but
they do not account for the increased level of dominance in
unmown meadows.
Munching molluscs complicate clover competition
Competition between
plants is a complex, multi-factorial affair as illustrated by
the work of
Hanley and Sykes (Plymouth and Southampton, pp. 1347–1353).
In experiments involving just two species,
Trifolium pratense and
T. repens, the authors have investigated the effects of
herbivory on interspecific competition. The two species were
initially grown separately. At 14 days,
T. pratense seedlings
were more than twice the size of
T. repens seedlings but were
also twice as acceptable to a herbivore, the snail
Helix aspersa.
Seedlings of each species were then grown together, 11 seedlings
of each per pot, in arrays that ensured that patterns of association
were constant. Different intensities of herbivory were achieved
by allowing different numbers of snails (0, 2, 5 or 10) access
to the pots overnight when the seedlings were 14 days old. Even
at the lowest density of snails, seedling mortality was over
50 %, and at 10 snails per pot over 90 % of the seedlings of
both species died. Surviving seedlings were grown for a further
106 days; plant biomass was then determined. In the control
pots (no snails)
T. pratense dominated with a biomass 10-fold
greater than
T. repens. However, herbivory caused major changes
in the balance between the two species. After exposure to two
snails per pot,
T. pratense was slightly (but not significantly)
dominant. In the five snails per pot treatment,
T. repens was
very dominant with a biomass more than 20 times greater than
that of
T. pratense. The seedlings of both species exposed to
the highest level of herbivory showed very much reduced biomass
(which was expected because of the high level of mortality at
14 days) with neither species dominating. So, even in this experiment
involving only two species it is clear that density of herbivores,
sensitivity to herbivores and innate differences in competitive
ability all contribute to the likelihood of plant recruitment
to a community.
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