Botanical Journal of the Linnean Society, 2002, 138, 117–122. With 10 figures
New perspectives on the mechanisms of chromosome
evolution in parasitic flowering plants
BATIA PAZY* and UZI PLITMANN
Department of Plant Sciences, The Alexander Silberman Institute of Life Sciences,
The Hebrew University of Jerusalem, Jerusalem 91904, Israel
Received May 2001; accepted for publication August 2001
In an attempt to find any association between chromosomal characters and parasitism, a cytological study of parasitic flowering plants in Israel has been carried out. While no such association was found, evidence for three levels
of chromosome evolution could be discerned: intra-chromosomal modifications, polyploidy per se, and genome
restructuring in polyploids. Our conclusions may serve as a paradigm of the multiple pathways of chromosomal
evolution in plants in general. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society,
138, 117–122.
ADDITIONAL KEY WORDS: dioecious plants – diploidization – holocentric (holokinetic) chromosomes – polyploidy – permanent translocation heterozygotes.
INTRODUCTION
Parasitic flowering plants, their physiology (including
a molecular survey of plastid genes and plastome) and
their impact on agriculture, have been investigated
extensively (e.g. Musselman, 1987; Press & Graves,
1995; Nickrent et al., 1997). Cytotaxonomical investigations have dealt mainly with chromosome numbers
or DNA content (e.g. Wiens, 1975; Barlow & Martin,
1984), or with particular phenomena in limited
groups, such as disturbed meiosis in Orobanche
(Moreno et al., 1979), or the cytogenetic basis of dioecy
in Viscum (Wiens & Barlow, 1975). We too have carried
out an intensive study, for several years, of the cytology of such plants (Pazy, 1988, 1997, 1998; Plitmann,
1991; Pazy et al., 1996; Plitmann & Joel, 1998; Pazy
& Plitmann, 1987,1991,1994, et al., 1996).
The modes of chromosome evolution in eukaryotes,
as based on cytological, molecular or genetic data,
have been discussed by many authors, from different aspects, e.g. duplicated genes and gene silencing (Lynch & Conery, 2000), holokinetic chromosomes
*Corresponding author.
E-mail: [email protected]
(Hughes-Schrader 1948; Wrensch et al., 1994), polyploidy and recurrent formation of polyploidy (Leitch &
Bennett, 1997), polyploidy and evolution of gender
dimorphism in plants (Miller & Venable, 2000).
In view of the latter aspects, together with previous
findings by our group and other workers, we are
approaching a comprehensive insight into the different mechanisms of chromosome evolution in this
group of parasitic flowering plants. In this article we
present a review of the major chromosomal features of
parasitic plants, based mainly on our observations on
material from Israel.
MATERIAL AND METHODS
Flowering specimens (one to several populations of
each) of seven genera were collected in Israel. The
genera sampled were: Cistanche Hoffmanns & Link.;
Cuscuta L.; Cynomorium L.; Cytinus L.; Orobanche L.;
Osyris L. and Viscum L. Vouchers are deposited in the
herbarium of The Hebrew University of Jerusalem.
There are 11 parasitic genera known to occur in Israel.
Two of the local genera (Cuscuta and Orobanche, both
holoparasites) are represented by as many as 12 and
11 species, respectively, but the other nine are represented by only one species (six genera) or two (three
genera). Material from four of the local genera was
© 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 138, 117–122
117
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B. PAZY and U. PLITMANN
not collected, these being Bellardia All., Parentucellia
Viv., Plicosepalus Tieghem (= Loranthus L., p.p) and
Thesium L.
Flower buds were fixed in the field in ethanol–acetic
acid (3:1), preserved in 70% ethanol, and then stained
in 2% acetocarmine. Chromosome examinations were
carried out in somatic and in pollen mother cells
of anthers. During the last 15 years, demographic
surveys and field observations have been carried out,
with an emphasis on the reproductive modes, fertility
and productivity, of these parasitic species.
MECHANISMS OF CHROMOSOME
EVOLUTION IN PARASITIC
FLOWERING PLANTS
No evident association could be found between parasitism and cytological characters. The four major chromosomal characters of the local parasitic plants do not
seem to be correlated, i.e. large chromosomes occur in
species with large numbers of chromosomes as well as
in species with small numbers, and holocentric chromosomes are present both in species with small as well
as with large numbers of chromosomes. However, we
could identify three levels of chromosomal evolution as
revealed from features of the parasites’ chromosomal
systems. These are: intra-chromosomal modifications,
polyploidy, and genome restructuring in polyploids.
INTRA-CHROMOSOMAL
MODIFICATIONS
Intra-chromosomal modifications may occur either at
the DNA level (nucleotide, gene or segment) or as epigenetic events, through duplications, deletions, inversions, silencing, etc. Dramatic cytological evidence of
a major modification in the basic structure of
the eukaryotic chromosome is the shift between diffuse and monocentric activity. The centromere and its
proteins (the kinetochore), the focus of many recent
studies (e.g. Pluta et al., 1995), are involved in cell
cycle checkpoint control, and are essential for chromosome segregation in mitosis and meiosis. Any
change in the structure of the centromere might be
crucial to the chromosomes’ functioning.
Although holocentric (holokinetic) chromosomes
occur in several plants and animals (Wrensch et al.,
1994), as for example in the well-studied and sequenced genetic model organism Caenorhabditis elegans
(Albertson & Thomson, 1982), their structure and
function are not yet understood fully. Despite their different organization, at least in C. elegans, they share
many features with monocentric chromosomes: they
have similar telomeric repeat sequences (TTAGGC,
Wicky et al., 1996) and their kinetochores have similar
trilaminar structure with similar microtubule density
(Albertson et al., 1997).
The ancestral nature of holocentric chromosomes
and inverted meiosis (a distinct mode of meiosis,
typical to holocentric chromosomes) has been suggested by several authors (e.g. Hughes-Schrader &
Schrader, 1961; Sybenga, 1981). The existence of
holocentric chromosomes in a Cuscuta species with
the lowest chromosome number in the genus (C.
babylonica, 2n = 8, Figs 5–8), as well as in six other
species with higher chromosome numbers of the same
subgenus (Pazy & Plitmann, 1987, 1991, 1994, 1995;
Garcia 2001), may support their early origin. Similar
low chromosome numbers also occur in other holocentric species, e.g. 2n = 6 in Luzula purpurea (Malheiros
et al., 1947) and 2n = 4 in Rhynchospora tenuis
(Vanzela et al. 1996). The shift between monocentric
and holocentric chromosomes apparently occurred
early in the course of evolution. In Cuscuta, the only
subgenus in which (only) holocentric chromosome
behaviour has been observed is the Old World subgenus Cuscuta Engelm. The two other subgenera have
monocentric chromosomes and evolved independently
in the Old World (Monogyna Engelm., Figs 1–4)
and in the New World (Grammica Yuncker, Pazy &
Plitmann, 1995). Chromosomal evolution within subgenus Cuscuta could occur through agmatoploidy (see
Luceòo & Guerra 1996).
POLYPLOIDY
Polyploidy (and dysploidy) has an important evolutionary role in eukaryotes in general, and it is a prominent process in plants in particular, as already
suggested by Stebbins (1950). Masterson (1994) estimated that 70% of all angiosperms are polyploids.
Although hybridization and spontaneous gene flow
between polyploids have been documented (e.g. in
a cytological survey of three generations in natural
introgression between polyploid Aegilops species, Pazy
& Zohary 1965), polyploids have been considered
previously as evolutionary dead-ends (Wagner, 1970).
However, recent studies, using genomic in situ hybridization (GISH, following Bennett et al., 1992), and
restriction fragment length polymorphism analysis
(RFLP, Moore et al., 1995) demonstrated that recurrent origins of polyploid species are the rule, not
the exception. Most taxonomically recognized polyploid species, many of them considered as ‘diploids’
(e.g. the cultivated corn), have been formed more than
once in different populations of the same diploid progenitor species (Soltis & Soltis, 1993, 1999). Gene flow
between polyploids might permit recombination and
serves as a significant factor in the production of
additional genotypes.
Apparently, polyploidy has also played a major role
in the evolution of the local parasitic plants: high chromosome numbers ranging between 2n = 8 and 2n = 56
© 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 138, 117–122
CHROMOSOME EVOLUTION IN PARASITIC FLOWERING PLANTS
119
Figures 1–8. Intra-chromosomal modifications: monocentric and holocentric chromosomes in Cuscuta species. Scale bars
= 10 mm. Figures 1–4. Monocentric chromosomes: nuclear divisions in Cuscuta monogyna (2n = 30). Fig 1. Mitotic
metaphase. Fig. 2. Anaphase. Fig. 3. Meiotic premetaphase I. Fig. 4. Premetaphase II. Figures 5–8. Holocentric chromosomes: nuclear divisions in Cuscuta babylonica (2n = 8). Fig. 5. Mitotic metaphase. Fig. 6 Anaphase: the sister chromatids
are situated parallel to the equatorial plane with no sign of localized centromere activity. Fig. 7. Meiotic first metaphase:
the chromatids are detached but remain associated end-to-end, rendering each bivalent a quadripartite configuration.
Fig. 8. Meiotic second metaphase, resembling that of a duplicated regular meiotic first metaphase of monocentric
chromosomes.
© 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 138, 117–122
120
B. PAZY and U. PLITMANN
in the Cuscutaceae, 2n = 24–60 in the Orobanchaceae
(Pazy, 1998), 2n = 32 in Cytinus hypocistis, and 2n = 38
in Osyris alba L. (Pazy & Plitmann, unpub.) are strong
indications supporting it. A bimodal karyotype (i.e. 18
large and 10 small chromosomes, Fig. 9) in Cynomoriaceae (Pazy et al., 1996; possibly as in the bimodal
karyotype of Milium montianum, as described by
Bennett et al., 1992) may serve as an indication of
allopolyploidy. Other parasitic taxa with high chromosome numbers, hence suspected to be polyploids,
belong to the Loranthaceae, Santalaceae, Olacaceae
and the scrophulariaceous genus Parentucellia. A
survey of the literature (e.g. Bolkhovskikh et al., 1969;
Cubero, 1996) reveals that the rate of probable polyploids among parasitic plants is at least twice that of
diploids.
GENOME
RESTRUCTURING IN POLYPLOIDS
Once in a common polyploid nucleus, intra- and
intergenomic rearrangements of parental diploid genomes are possible, and the divergent genomes may
undergo a rapid and extensive reorganization. Additional copies of genes compensate for the loss or
altered expression of genes or chromosomes and serve
as buffer against possible deleterious consequences
(e.g. Wendel, 2000). Extensive gene silencing in polyploids, possibly through transposable elements (e.g.
Matzke & Matzke, 1998), will eventually lead to chromosome diploidization, and the genome will no longer
be structured as an allopolyploid.
Nontheless, diploidization is not achieved in all
polyploids. Common irregularities in meiosis of the
local Orobanchaceae species observed by us (Pazy
1998) as well as by other authors in other regions
(e.g. Greilhuber & Weber, 1975; Cubero, 1996), point
to the possibility of diploidization failure. This situation is apparently fixed in the Orobanchaceae (and
other polyploids) by parthenogenesis (Jensen, 1951)
or apomixis (Pazy, 1998; Plitmann & Joel, 1998),
i.e. through vegetative reproduction or agamospermy.
Such coexistence of asexual and sexual plants may
provide alternative strategies for better exploitation of
the environment. Similar multiple reproductive modes
occur also in Cuscutaceae (Plitmann, 1991), as well
as in many other parasitic species (Kuijt, 1969; own
observations). Miller & Venable (2000) also suggest
that polyploidy may be an important trigger for
gender dimorphism, which ‘operates by disrupting
self-incompatibility (of the parental diploids) and
leading to inbreeding depression’.
Recent studies using chromosome-specific fluorescent markers have identified several intergenomic
translocations in allopolyploids (Leitch & Bennett,
1997). Similar changes might lead to gender dimorphism. Gender dimorphism is present in four fami-
Figures 9,10. Scale bars = 10 mm. Fig. 9. Polyploidy: a possible combination of two ancestral genomes (allopolyploidy)
expressed in the bimodal karyotype in Cynomorium coccineum (2n = 28), with 10 very short chromosomes (5 bivalents)
and 18 much larger (9 bivalents), in late diakinasis. Fig. 10. Genome restructuring in polyploids: gender dimorphism in
Viscum cruciatum (2n = 20, chromosomes among the largest in the flowering plants): a permanent translocation heterozygous male plant, showing a chain of eight chromosomes (alternate orientation, including one detached chromosome)
and six bivalents at MI.
© 2002 The Linnean Society of London, Botanical Journal of the Linnean Society, 2002, 138, 117–122
CHROMOSOME EVOLUTION IN PARASITIC FLOWERING PLANTS
lies (Cynomoriaceae, Loranthaceae, Rafflesiaceae and
Santalaceae) out of the seven families of the local
parasitic flowering plants (and in half of all parasitic
plant families, according to Kuijt 1969), apparently
reaching its evolutionary peak in the local dioecious
Viscum cruciatum. In the latter species, a permanent translocation complex is maintained through
sex association, i.e. the male plants are translocation
heterozygotes and consistently have an association of
eight chromosomes and six bivalents (Fig. 10, see also
Pazy, 1988), and the female plants are structurally
homozygous and show ten bivalents (Barlow et al.,
1978). Translocation heterozygosity is also characteristic of male plants of other species of Viscum (Wiens
& Barlow, 1975; Barlow et al., 1978).
Allen (1940) listed 70 species (of 25 families) of
angiosperms in which sex chromosomes were detected
(see also Westergraad, 1958). In Viscum, permanent
translocation in males, in which several chromosomes
are involved, and homozygosity in females may represent an intermediate stage in the evolution of
sex-determining mechanisms (Charlesworth, 1991),
perhaps fixed by a balanced lethals system.
All in all, the local parasitic flowering plants,
sharing the same mode of life, may serve as a paradigm of chromosomal evolution in other plant groups
in general.
ACKNOWLEDGEMENT
Thanks are due to Shimrit Lahav-Ginott for her
valuable help.
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