Annals of Botany 2009 103(7):iii; doi:10.1093/aob/mcp094
© 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
On the origin of species by varietal selection
As we approach the
150th anniversary of the publication of Charles Darwin's seminal
work, it is appropriate that we again discuss speciation mechanisms.
Speciation often occurs when populations become separated from
each other. Separation may be geographically obvious but it
may also take more subtle forms. One such example is seen in
the preferences for different host plants in different populations
of parasitic species, as has been studied by
Thorogood et al. (Bristol University and Natural History Museum, London, pp. 1005–1014).
Orobanche minor is a parasite with a very broad host range,
but within that range specific varieties or sub-species of the
parasite may exhibit specific host preferences. The authors
have examined the parasitic behaviour of
O. minor var.
minor and
O. minor ssp.
maritima that usually infect, respectively,
red clover and sea carrot. In reciprocal infection experiments,
seeds of
O. m. maritima germinated in the presence of both host
species; the germination percentage was much higher with sea
carrot than with red clover and only with sea carrot was there
any attachment of the seedlings to the host root.
Orobanche m. maritima therefore did not infect red clover, showing an
early selection of its normal host. Surprisingly,
O. m. minor showed much better germination and attachment with sea carrot
than with its usual host, red clover. However, its ‘performance’
as a parasite, as measured by parasite biomass, by the formation
of flowers and by effects on host biomass, was much better with
red clover than with sea carrot. Thus in
O. m. minor, host selection
occurs at a later stage than is seen with
O. m. maritima. These
data, combined with the authors' earlier demonstration that
the two varieties exhibit genetic differences, indicate speciation
in progress. Further, because
O. minor is a self-fertile species,
genetic differences between varieties will be reinforced by
the strong tendency to inbreeding, thus accelerating the speciation
process.
Sediment CO2 satisfies several submerged species
Submerged aquatic
plants have a problem in obtaining CO
2 for photosynthesis. Although
CO
2 concentration in water is much higher than in air, the low
gas diffusion rate in water more than cancels this out. Some
species adapt to submergence by induction of CAM, effectively
a CO
2-concentrating mechanism (e.g.
Littorella uniflora: Groenhof
AC, Smirnoff N, Bryant JA. 1988.
Journal of Experimental Botany 39: 353–361). Another adaptation shown by plants of isoetid
life form is the possession of aerenchyma, which brings CO
2 from the sediment via the roots to the photosynthetic organs.
The question asked by
Winkel and Borum at the University of Copenhagen (pp. 1015–1023) is whether non-isoetid submerged species can also use sediment
CO
2. They selected four non-isoetid aquatic species (
Lilaeopsis macloviana,
Ludwigia repens,
Vallisneria americana,
Hydrocotyle verticillata) and, for comparison, one isoetid (
Lobelia dortmanna).
Plants were placed in two-compartment split chambers; the CO
2 concentration around the roots was manipulated independently
of that around the shoots. Thus rates of photosynthesis using
CO
2 supplied to the roots at various concentrations could be
compared with the rates obtained when leaves were supplied with
CO
2 at different concentrations. Of the five species, only
Hydrocotyle was totally unable to use root-supplied CO
2. Of the others,
Lobelia was, as expected, the most efficient, but two of the
non-isoetids,
Lilaeopsis and
Vallisneria, were also able to
obtain up to 75 % of their CO
2 requirement from the sediment
when the CO
2 concentration in the water was in equilibrium with
air. All four users of CO
2 from sediment have low shoot : root
area ratios, in contrast to
Hydrocotyle in which the ratio is
much higher. However, utilization of CO
2 via the roots in
Ludwigia is much lower than would be expected from its shoot : root ratio.
Use of
14C suggested that in
Ludwigia, utilization of CO
2 from
sediment is limited by rates of uptake and/or transport. In
conclusion, sediment CO
2 is likely to play a much larger role
in the carbon economy of submerged plants than had been suspected.
Ants provide service for flowers on ground surface
‘Biological
oddities have traditionally attracted the attention of naturalists’,
write
de Vega et al., at Sevilla, Spain (pp. 1065–1075).
I am sure that we all agree, but I would extend ‘naturalist’
to include anyone with a strong interest in biology. It is indeed
fascinating to see what evolution can lead to in terms of life
form, life style, habitat use and so on. Thus we consider those
most bizarre parasitic plants that live as endophytes, totally
within their hosts except when flowering. Flowers are produced
from within the host's root systems and appear at ground level,
the only sign visible to the ‘outside world’ of
the parasite within its host.
Rafflesia is the obvious example,
but in fact there are four different plant families of endophytic
parasites. The authors worked with a Mediterranean species,
Cytinus hypocistis, and wanted to identify its pollination agents.
This species is self-fertile but is monoecious, so self-pollination
requires pollen transfer from flower to flower, albeit on the
same plant. Long-term, detailed and patient field observations
were made, showing that ants were responsible for 97·4
% of floral visits. Surprisingly, very few flying insects visited
the flowers and only one of these, the fly
Oplisia aterrima,
was a pollinator. Five species of ant were regular visitors
and another five were less regular. Ants were observed carrying
pollen between flowers; pollen viability was slightly reduced
after contact with ants; nevertheless fruit set was between
80 and 90 % in plants from which other pollinators had been
excluded. This may be compared with the 96–99 % seen in
open-pollinated plants, confirming that despite the infrequency
of its visits,
Oplisia aterrima did participate in pollen transfer
(wind-pollination did not occur). However, it is abundantly
clear that ants are the major pollinators, being rewarded with
nectar during the process. This contrasts with South African
Cytinus species, which are pollinated by small mammals.
Retreat from SI gives some measure of success
The reversion from
self-incompatibility to self-fertility has occurred many times
in the evolution of angiosperms. This reversion is totally dependent
on the breakdown of the self-incompatibility system, based on
recognition of the products of self and non-self S-alleles.
Further, there are several species in which there is partial
self-incompatibility. An example studied by a Canadian–Mexican
group,
Ferrer et al. (pp. 1077–1089) is
Flourensia cernua (Asteraceae), a shrub of the Chihuahuan desert, Mexico. Space
permits discussion of only a limited part of the authors' comprehensive
and fascinating paper. In two populations of
F. cernua, one
from high and one from low scrub density, different levels of
self-incompatibility were detected by comparing seed set in
hand self-pollinated and hand cross-pollinated plants. Strongly
self-incompatible (SI) plants made up approx. 50 % of the population;
at the other end of the scale, 20 % were self-compatible (SC).
However, the situation was more complex than this. Strongly
SI plants set very few seeds, either in hand cross-pollination
or in open pollination. This may indicate pollen limitation,
possibly based on a limited range of S-alleles in the population.
In the other classes, seed set was higher after hand cross-pollination
than after open pollination. Interestingly and perhaps unexpectedly,
seed set in open-pollinated plants was greater in the low density
scrub than in the high density: pollen ‘quality’
was higher. Further, in plants in which there was a total or
partial breakdown of self-incompatibility,
out-crossed seed
set, despite the high level of self-compatibility, was higher
than in strongly SI plants. There were also effects on seed
fitness: germination rates were lower in seeds arising from
open pollination in the SI plants than in the partially or totally
SC plants. The authors suggest that partial self-incompatibility
is selected for in
F. cernua but that nevertheless, the level
of inbreeding is relatively low. Partial breakdown of self-incompatibility
may thus be a strategy to increase reproductive output especially
when colonizing new habitats.
CiteULike 
Connotea 
Del.icio.us What's this?
Related articles in Ann Bot:
- Host-specific races in the holoparasitic angiosperm Orobanche minor: implications for speciation in parasitic plants
- C. J. Thorogood, F. J. Rumsey, and S. J. Hiscock
Ann Bot 2009 103: 1005-1014.
[Abstract]
[Full Text]
- Use of sediment CO2 by submersed rooted plants
- Anders Winkel and Jens Borum
Ann Bot 2009 103: 1015-1023.
[Abstract]
[Full Text]
- The ant-pollination system of Cytinus hypocistis (Cytinaceae), a Mediterranean root holoparasite
- Clara de Vega, Montserrat Arista, Pedro L. Ortiz, Carlos M. Herrera, and Salvador Talavera
Ann Bot 2009 103: 1065-1075.
[Abstract]
[Full Text]
- Effect of variation in self-incompatibility on pollen limitation and inbreeding depression in Flourensia cernua (Asteraceae) scrubs of contrasting density
- Miriam M. Ferrer, Sara V. Good-Avila, Carlos Montaña, César A. Domínguez, and Luis E. Eguiarte
Ann Bot 2009 103: 1077-1089.
[Abstract]
[Full Text]
- ContentSnapshots
Ann Bot 2009 103: i.
[Extract]
[Full Text]
