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INTRODUCTION |
Arabidopsis has been
enormously successful in nature as a colonizer of disturbed areas, as
is evident from its widespread distribution in the northern hemsiphere.
Although most physiological and molecular research studies of
Arabidopsis have utilized only one of two northern European ecotypes
(Columbia or Landsberg erecta [Ler]), ecotypic accessions
are available from around the world, including North America, Europe,
Asia, Australia, and Africa. To date, these accessions have been used
primarily in screening for phenotypes that are especially resistant to
biotic or abiotic stresses, such as bacteria (Debener et al., 1991
),
nematodes (Sijmons et al., 1991), powdery mildew (Parker et al., 1996),
or phosphate deficiency (Narang et al., 2000). The long-term objectives
of these studies are to isolate and characterize the genes that confer resistance to a given stress or pest, and to introduce them
transgenically into other Brassicaceae, if not unrelated species.
However, the natural allelic variation of Arabidopsis is also
increasingly being exploited by a second group of biologists who have
less applied objectives in mind. These researchers aim to use
Arabidopsis ecotypes to bridge the gap between field ecology and plant
molecular biology. This month's The Hot and the Classic
highlights some of the recent advances that have been made using
Arabidopsis as an ecological model.
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The Role of Humans and Refugia in Arabidopsis
Biogeography |
Given that Arabidopsis reproduces mostly by self-fertilization, it
is not too surprising that Arabidopsis generally shows little genetic
heterogeneity within any given population in the wild. Between
populations, however, there is considerably more genetic heterogeneity,
the studies of which are revealing new and unexpected details of
Arabidopsis's natural biogeography.
For example, Sharbel et al. (2000) analyzed 79 amplified fragment
length polymorphism markers in 142 accessions from Arabidopsis's native range. The spatial patterns of genetic variation that were noted suggest that Arabidopsis colonized central and northern Europe
from Asia and from Mediterranean Pleistocene refugia (Iberia and Asia),
a trend which has been identified in other species. Because of such
palaeoclimatic advances and retreats as well as human interference,
there appears to have been substantial historical recombination in the
Arabidopsis genome such that accessions do not conform to a typical
tree-like, bifurcating pattern of evolution.
In a second study aimed at elucidating the intraspecific phylogenetic
relationships between Arabidopis ecotypes from widely separated areas
of the world, Vander Zwan et al. (2000) studied 18 populations of
Arabidopis using polymorphic DNA and morphologic analyses. A surprising
detail, and one with considerable cautionary import, is that the
ecotype Kashmir, the only Indian representative in the Arabidopsis
Resource Centers of both Europe and North America, appears to have
originated in Europe (probably near Loch Ness in Scotland), not India.
The authors speculate that "Kashmir" is probably the descendant of
a contaminating Scottish ecotype that hitchhiked to India in a shipment
of grain within the last 150 years. Their results also confirm the
commonly held premise that North American Arabidopsis populations also
came from Europe relatively recently, and show little heterogeneity.
For example, two North American ecotypes, Martha's Vineyard (MA) and
Yosemite (CA), although collected from two very different habitats and geographically separated by thousands of kilometers, are
virtually identical to each other both genetically and morphologically.
Thus, the story that is emerging is that Arabidopis's relatively
recent success in the wild has occurred in the wake of environmental upheavals wrought by Homo sapiens. Transglobal commerce and
migrations have led to the unintentional introduction of Arabidopsis
ecotypes to new and distant locales, thereby obscuring the pattern of
its natural phylogenetic spread.
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Latitudinal Variations in Relative Growth Rate
(RGR) |
As Arabidopsis has spread northward across the northern
hemisphere, one of the stronger evolutionary pressures that it has faced is adapting to the local environmental conditions of which latitude is an important determinant. For example, harsh and infertile conditions normally produce inherently small ecotypes, which may also
be correlated with short development cycles, growing seasons, and life
spans. To investigate genetic variation in plant size and RGR along a
latitudinal gradient, Li et al. (1998) selected 40 ecotypes of
Arabidopsis selected from a wide range of latitudes (from 16°
north-63° north). Greenhouse-raised plants from high latitude stocks
tended to have smaller plant size in terms of seed size, cotyledon
width, rosette size, number of rosette leaves, size (leaf area) of the
largest leaves, total leaf area, and total dry weight per plant than
those from low latitudes. There was also significant ecotypic
variation, with RGR being negatively correlated with latitude. Although
significant, the variation between the RGRs of Arabidopsis ecotypes was
small compared with other species that have been studied previously.
The authors propose that RGR may be a conservative trait, whose
variation is constrained by the trade-off between its physiological and
morphological components. One wonders, however, whether the small range
of the ecotypic variations in RGR might not also be attributable to
Arabidopsis's recent and complex pattern of spread.
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Variations in Seed Allocations |
Resource availability, competition, and genetic difference are
among the main factors causing size variation in plant populations. These factors also have great effects on the variation in reproductive allocation. Alonso-Blanco et al. (1999) determined that the accession Cape Verde Islands (Cvi) yielded 40% fewer seeds than did the accession Ler, but these seeds were almost twice as heavy. The Ler/Cape
Verde Islands seed size difference involved changes in the cell number
and cell size of the seed coat and the embryo. Cell number variation
was controlled mainly by maternal factors, whereas non-maternal allelic
variation mostly affected cell size. These ecotypic differences may
provide molecular insights into a fundamental question in population
ecology: Under what circumstances is it beneficial for an organism to
invest its accumulated resources strongly in each of a few offspring or
sparingly in each of numerous offspring?
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Vernalization |
By comparing different ecotypes, molecular biologists are
providing new insights into the physiological mechanisms that underlie the natural ecotypic variations of Arabidopsis. For example,
vernalization, the acceleration of flowering by a long period of cold
temperature, ensures that many plants overwinter vegetatively and
flower in spring. In Arabidopsis, allelic variation at the FRIGIDA
(FRI) locus is a major determinant of natural variation in
flowering time. Dominant alleles of FRI confer late flowering, which is reversed to earliness by vernalization. Johanson et al. (2000) cloned
FRI and analyzed the molecular basis of the allelic variation. Most of
the early flowering ecotypes analyzed carry FRI alleles containing one
of two different deletions that disrupt the open reading frame.
Loss-of-function mutations at FRI thus have provided the basis for the
evolution of many early flowering ecotypes.
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Response to Shading |
Plants shaded by neighbors or overhead foliage experience both a
reduction in the ratio of red to far red light (R:FR), a specific cue
perceived by phytochrome, and reduced photosynthetically active
radiation, an essential resource. Dorn et al. (2000) tested the
adaptive value of plasticity to crowding and shading in Arabidopsis by
exposing 36 inbred families from four natural populations to four
experimental treatments: (a) high density and full sun, (b) low density
and full sun, (c) low density and neutral shade, and (d) low density
and low R:FR-simulated foliage shade. Genotypic selection analysis
within each treatment revealed strong environmental differences in
selection on plastic life history traits. Contrary to expectation, no
evidence was found for adaptive plasticity to density, but both
adaptive and maladaptive responses to foliage shade were noted. In
general, phytochrome-mediated plasticity to the R:FR cue of foliage
shade was adaptive and counteracted maladaptive growth responses to
reduced photosynthetically active radiation.
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INTRODUCTION
The Role of Humans...
