Biological Journal of the Linnean Society, 2009, 98, 187–197. With 3 figures
Rapid diversification of mating systems in ciliates
bij_1250
187..197
SUJAL S. PHADKE and REBECCA A. ZUFALL*
Department of Biology and Biochemistry, University of Houston, Houston, TX 77204, USA
Received 9 December 2008; accepted for publication 6 February 2009
Ciliates are a diverse group of microbial eukaryotes that exhibit tremendous variety in several aspects of their
mating systems. To understand the evolutionary forces driving mating system diversification in ciliates, we use a
comparative approach synthesizing data from many ciliate species in light of recent phylogenetic analyses.
Specifically, we investigate the evolution of number of mating types, mode of mating type inheritance, and the
molecular determinants of mating types across the taxonomic diversity of ciliates, with an emphasis on three
well-studied genera: Tetrahymena, Paramecium, and Euplotes. We find that there have been many transitions in
the number of mating types, and that the requirement of nuclear reorganization may be a more important factor
than genetic exchange in determining the optimum number of mating types in a species. We also find that the
molecular determinants of mating types and mode of inheritance are evolving under different constraints in
different lineages of ciliates. Our results emphasize the need for further detailed examination of mating systems
in understudied ciliate lineages. © 2009 The Linnean Society of London, Biological Journal of the Linnean
Society, 2009, 98, 187–197.
ADDITIONAL KEYWORDS: Euplotes – evolution – inheritance – mating types – Paramecium – Tetraymena.
INTRODUCTION
Broadly defined, mating systems determine which
individuals in a population will engage in genetic
exchange, how that exchange will occur, and what
will be the resulting genotype of the offspring. Under
this definition, mating systems are clearly central to
understanding how evolution occurs within a species.
Additionally, changes in mating systems may have
important implications for gene flow and speciation.
Ciliates are a morphologically and genetically
diverse group of microbial eukaryotes. The remarkable diversity in this group is evident in genetic
code usage (Lozupone, Knight & Landweber, 2001),
developmental genome rearrangements (Jahn &
Klobutcher, 2002; Yao, Duharcourt & Chalker, 2002;
Zufall, Robinson & Katz, 2005), surface antigen composition (Doerder et al., 1996), and many aspects of
mating systems. Research on ciliates has historically
resulted in major advances in biology; for example,
discoveries of self-splicing RNA (Kruger et al., 1982)
*Corresponding author. E-mail: [email protected]
and telomerase (Blackburn & Gall, 1978). The goal of
the present study was to demonstrate that studying
mating systems in ciliates may likewise lead to major
advances in our understanding of how sexual systems
evolve. The diversity found in all aspects of mating
systems in ciliates makes this an ideal group in which
to study mating system evolution. In particular, we
aim to elucidate how mating systems have evolved in
ciliates by synthesizing data on the various aspects of
these systems in a comparative context.
Mating systems in ciliates have been studied since
1937 when Sonneborn first described the presence of
mating incompatibility groups within Paramecium
aurelia (Cl: Oligohymenophorea) (Sonneborn, 1937).
Subsequent to this discovery, mating types were found
in almost all ciliates examined (but see also Nanney &
McCoy, 1976) and various aspects of mating systems
have been studied broadly in ciliates (Dini & Nyberg,
1993; Miyake, 1996). As a result of recent molecular
phylogenetic analyses of ciliates, we are now able to
follow the evolution of various aspects that characterize mating systems across the ciliate phylogeny.
We examined the evolution of number of mating
types, mating type determinants, and mating type
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
187
188
S. S. PHADKE and R. A. ZUFALL
inheritance. In general, we found that each of these
aspects is evolving rapidly within ciliates. We also
discuss trends in mating system evolution within
specific lineages of ciliates in light of previous
hypotheses.
SEX AND REPRODUCTION IN CILIATES
In ciliates, sex (i.e. genetic exchange) is decoupled
from reproduction (i.e. increase in numbers) (Fig. 1).
Genetic exchange occurs via conjugation between
cells of compatible mating types, which exchange
gametic nuclei. In most ciliates, genetic exchange is
isogamous and reciprocal (i.e. equal sized meiotic
products are exchanged between the mating part-
ners). Within ciliates, there have been transitions
from isogamy to anisogamy (unequal sized gametes
with unidirectional transfer) in suctorian ciliates
(class Phyllopharyngea; Fig. 2) (e.g. Tokophrya infusionum and T. lemnarum; Sonneborn, 1978; Miyake,
1996) and also in some peritrichous ciliates (class
Oligohymenophorea) (e.g. Vorticella microstoma;
Finley, 1943; Grell, 1973). Reproduction is asexual, in
the form of binary fission in most ciliates. Some
stalked ciliates, including the anisogamous suctorian
and peritrichous ciliates, divide by budding.
In ciliates, as well as in other organisms (e.g. fungi;
Fraser & Heitman, 2003), which individuals in a
population can engage in genetic exchange is determined by mating types. A single ciliate species may
Reciprocal fertilization
during conjugation
Macronuclear development
Exconjugants
Asexual reproduction
by fission
Synclonal:
All progeny have
same phenotype;
Genetic inheritance
Cytoplasmic:
Progeny express
parental phenotype;
Epigenetic inheritance
Caryonidal:
Stochastic determination
of phenotype;
Epigenetic inheritance
Figure 1. Life cycle (simplified) and patterns of inheritance. Genetic exchange in ciliates occurs via reciprocal fertilization during conjugation between individuals with compatible mating types. Haploid nuclei are exchanged and the
stationary gametic nucleus fuses with the migratory gametic nucleus resulting in identical diploid nuclei in the
exconjugants. The diploid nucleus then undergoes mitoses and the macronucleus differentiates from the micronucleus via
genome rearrangements that result in DNA loss and chromosomal fragmentation and amplification. This type of genetic
exchange and macronuclear development can result in three different patterns of mating type inheritance.
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
CILIATE MATING SYSTEM EVOLUTION
189
Metopus palaeformis
61/66/76
+
Nyctotherus ovalis
100/
98/100
Nyctotheroides deslierresae
Metopus contortus
Isotricha intestinalis
59/<50/-
Epidinium caudatum
100/100/99
70/54/-
100/87/82
+
Ophryoscolex purkynjei
+
Diplodinium denatum
*Dileptus sp.
Didinium nasutum
Obertrumia georgiana
100/54/<50
64/<50/<50
Pseudomicrothorax dubius
Furgasonia blochmanni
Chilodonella uncinata
+
Trithigmostoma steini
Ephelota sp.
+
93/-/-
+
70/57/64
100/99/100
Heliophrya erhardi
Tokophrya lemnarum
Discophrya collini
84/-/-
Coleps hirtus
+
95/71/76
62/<50/
<50
100/67/51
Coleps sp.
Paramecium tetraurelia
53/-/-
+
76/-/62
Tetrahymena thermophila
+
74/<50
/<50
Glaucoma chattoni
97/-/64
Ophryoglena catenula
Anophyroides haemophila
66/<50/-
Prorodon teres
+
Prorodon viridis
Bursaria truncatella
Bresslaua vorax
100/100/99
+
Colpoda inflata
Pseudoplatyophrya nana
Platyophrya vorax
*Diophrys oligothrix
Strombidium purpureum
*Stylonychia lemnae
55/<50
/57
100/88/73
69/-/<50
100/
98/
100
91/<50/51
Oxytricha granulifera
Halteria grandinella
Euplotes crassus
*Aspidisca steini
51/-/86
+
+
Loxodes striatus
Loxodes magnus
Tracheloraphis sp.
Eufolliculina uhligi
+
100/94/100
Stentor coeruleus
97/61/75
100/87/100
79/
56/63
Blepharisma americanum
Binary mating system
Gruberia sp.
Spirostomum ambiguum
Multiple mating system
Climacostomum virens
Figure 2. Ciliate phylogeny and ancestral character state reconstruction of mating type number. A phylogeny of the
phylum Ciliophora was constructed based on small subunit ribosomal DNA (SSU rDNA) using the same species as
employed in the study by Riley & Katz (2001) plus additional taxa for which we have mating system data. Trees were
built using Bayesian, maximum likelihood, and maximum parsimony criteria using MrBayes (Huelsenbeck & Ronquist,
2001) and PAUP* (Swofford, 2003) with models selected by MODELTEST (Posada & Crandall, 1998) as implemented in
GENEIOUS (Drummond et al., 2008). The phylogeny shown is the Bayesian topology with node support indicated as
Bayesian posterior probability/maximum likelihood bootstrap/maximum parsimony bootstrap. +, 100% support with each
method; -, node found in the Bayesian analysis, but not other analyses. Where data were not available for both SSU rDNA
and mating type number, an asterisk (*) indicates that the phylogeny was constructed with a different species than that
for which mating type number is shown. Genbank accession numbers for sequences used in this analysis are provided in
the Supporting information (Table S1). Mating type systems are mapped to the tree as either binary, two mating types
in a species (indicated by an open circle), or multiple, greater than two mating types (indicated by a closed circle).
Maximum likelihood ancestral character state reconstruction was performed in MESQUITE (Maddison & Maddison,
2008). Pie diagrams represent relative likelihoods of a state at each node.
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
190
S. S. PHADKE and R. A. ZUFALL
have anywhere from two to 12 or more (e.g. 100
for Stylonychia mytilus; Ammermann, 1982) mating
types (Table 1). Two cells having non-identical mating
types can undergo successful conjugation, which
involves exchange of haploid gametic nuclei and subsequent developmental differentiation (Fig. 1). A few
species are capable of mating within a clonal line,
which may be a result either of mating type switching
or the ability to mate with individuals of the same
mating type (homotypic mating).
MATING TYPE NUMBER
Understanding the factors that control the number of
mating types (or sexes) in a species is a long standing
issue in evolutionary biology (Power, 1976; Hurst,
1996; Whitfield, 2004). Ciliates exhibit a large range
of variation in number of mating types between
species. This provides the opportunity to determine
whether there is a trend towards an increase or
decrease in the number of mating types and whether
correlations between mating type number and other
traits may constrain or enhance the evolution of new
mating types.
MANY
TRANSITIONS IN NUMBER OF MATING TYPES
Previous studies have suggested that there might be
an evolutionary trend towards an increase in the
number of mating types in ciliates (Miyake, 1996).
This trend is expected if there is selection for an
increase in the probability of mating between any
two individuals in a population. For example, with
only two mating types in a population, if mating
type frequencies are equal, one half of the population
is unavailable for mating by any given individual.
By increasing the number of mating types in a population, the chances of finding a compatible mate are
increased. This may be similar to what is found in
some plant incompatibility systems where there is
a trend toward increasing the number of self incompatibility alleles driven by negative frequencydependent selection (Charlesworth et al., 2005).
When increased genetic exchange is favoured, an
increase in number of mating types may be
favoured.
To determine whether there is a trend in changes in
number of mating types throughout ciliate evolution,
we looked across the whole ciliate phylogeny at the
distribution of species with binary (two) or multiple
(more than two) mating types (Fig. 2). We see that
binary and multiple mating type systems are scattered across the phylogeny and there apparently have
been many changes that both increase and decrease
mating type number. Maximum likelihood ancestral
character state reconstruction suggests that the
ancestor of all ciliates had a binary mating system.
However, given the lack of mating system data in
early diverging lineages, we present this result with
caution. This issue will be revisited when more data
on ciliates in understudied classes become available.
We also tested the hypothesis of increasing
numbers of mating types at a finer phylogenetic scale,
within the genera Tetrahymena and Euplotes (Fig. 3).
In Tetrahymena, we see that species closely related to
Tetrahymena borealis tend to have more mating types
than species closely related to Tetrahymena australis.
Mating type number is variable in both of these
groups; however, there is no indication of a trend
to increase or decrease mating type number. In
Euplotes, there is also variation in the number of
mating types between species; however, there is no
trend toward increasing number of mating types.
With the exception of Paramecium bursaria, all
species that have been studied in Paramecium have
two mating types. Nanney (1980) demonstrated
that the observed pattern in the number of mating
types of P. bursaria reflects gene duplication at the
mating type locus. We examined the possibility that
the whole genome duplications in the Paramecium
lineage may explain the number of mating types
present in these species. However, considering the
apparent lack of similar increase in the mating type
number in the Paramecium aurelia species complex
following the recent genome duplications (Aury et al.,
2006), we present the hypothesis with caution.
CORRELATES
OF MATING TYPE NUMBER
As shown above, the number of mating types in a
species may not be a response to selection for increasing the probability of finding a compatible mate; thus,
it remains unclear what does control the evolution of
mating type number in a species. We examined two
additional features of ciliate mating systems, pheromone secretion and autogamy, to determine whether
either of these might influence the evolution of
mating type number.
The molecules that determine mating type are
typically protein pheromones (with a notable exception in Blepharisma; see below). These molecules can
be either secreted or cell-bound. One hypothesis suggests that the evolution of multiple mating types may
be constrained in species with secreted mating pheromones. Pheromone secretion has been demonstrated
in Blepharisma species, which have binary mating
type systems (Sugiura & Harumoto, 2001; Sugiura
et al., 2005). However Paramecium species, which
also have binary mating type systems, do not secrete
pheromones (Table 1). Pheromone secretion appears
to be ancestral in the genus Euplotes, and is lost in
later diverging species (Vallesi et al., 2008); however,
there is no correlation with mating type number.
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
CILIATE MATING SYSTEM EVOLUTION
191
Table 1. Mating system characteristics: number of mating types, mode of mating type inheritance, ability to undergo
autogamy, and whether mating type determinants are secreted or cell-bound
Mating type
number
Species
Inheritance
Autogamy
Secretion
References
5
5
7
10
Synclonal
Synclonal
Synclonal
Synclonal
NA
Yes*
Yes*
NA
NA
No
No
Yes
Euplotes patella
Euplotes raikovi
6
12
Synclonal
Synclonal
No
NA
Yes
Yes
Euplotes vannus
5
Synclonal
Yes*
No
Beale (1990); Miyake (1996)
Dini (1984)
Dini (1984)
Heckmann & Kuhlmann
(1986); Miyake (1996)
Kimball (1943); Siegel (1956)
Miceli, Luporini & Bracchi
(1981); Raffioni et al. (1992)
Gates (1990)
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2, 4, 8¶
2
2
2
Caryonidal
Cytoplasmic
Caryonidal
Cytoplasmic
Caryonidal
Cytoplasmic
Cytoplasmic
Cytoplasmic
Caryonidal
Cytoplasmic
Caryonidal
Cytoplasmic
Synclonal
Synclonal
Synclonal
Synclonal
Synclonal
Caryonidal/
Cytoplasmic
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Inducible
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Euplotes
Euplotes
Euplotes
Euplotes
Euplotes
aediculatus
crassus
minuta
octocarinatus
Paramecium
Paramecium aurelia complex
Paramecium primaurelia
Paramecium biaurelia
Paramecium triaurelia
Paramecium tetraurelia
Paramecium pentaurelia
Paramecium sexaurelia
Paramecium septaurelia
Paramecium octaurelia
Paramecium novaurelia
Paramecium decaurelia
Paramecium undecaurelia
Paramecium dodecaurelia
Paramecium tredecaurelia
Paramecium quadecaurelia
Paramecium bursaria
Paramecium caudatum
Paramecium multimicronucleatum
Paramecium sonneborni
Tetrahymena
Sonneborn (1974a, 1975)
Miyake (1996)
Kimball (1943)
Kimball (1943)
Siegel & Larison (1960)
Tsukii (1988)
Tsukii (1988); Giese (1941)
Tsukii (1988)
Sonneborn (1974b); Nanney &
McCoy (1976)
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
americanis
australis
borealis
capricornis
canadensis
cosmopolitanis
elliotti
hegewischi
hyperangularis
malaccensis
9
3
7
4
5
3
0†
8
4
6
Synclonal
NA
Caryonidal
NA
Caryonidal
NA
Amicronucleate
Synclonal
Synclonal
Caryonidal
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
Tetrahymena
Tetrahymena
Tetrahymena
Tetrahymena
pigmentosa
pyriformis
thermophila
tropicalis
3
0†
7
5
Synclonal
Amicronucleate
Caryonidal
Caryonidal
No
No
No
No
No
No
No
No
2
2
2
2
2
3
NA
Synclonal
Synclonal
Synclonal
Synclonal
Synclonal/
Caryonidal
NA
NA
Yes
Yes
Yes
Yes
NA
NA
Yes
Yes
Yes
Yes
No**
Dini, Bracchi & Gianni (1987)
Miyake (1996)
Sugiura et al. (2005)
Miyake (1996)
Miyake (1996)
Yudin & Uspenskaya (2007)
NA
NA
Borror (1980)
Aspidisca sp.
Blepharisma americanis
Blepharisma japonicum
Blepharisma musculus
Blepharisma stoltei
Dileptus anser
Diophrys sp.
ⱖ 3‡
Phillips (1969)
Phillips (1969)
Simon & Orias (1987)
Simon & Nanney (1984);
Simon & Orias (1987)
Simon & Orias (1987)
Simon & Orias (1987)
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
192
S. S. PHADKE and R. A. ZUFALL
Table 1. Continued
Mating type
number
Species
Glaucoma scintillans (US)
Glaucoma scintillans (Japan)
Glaucoma chattoni
Oxytricha bifaria
Oxytricha granulifera
Stentor coeruleus
Stylonychia putrina
Stylonychia mytilus
Tokophrya lemnarum
5
8
5
3
9
2§
5, 11, 15¶
100
2
Inheritance
Autogamy
Secretion
References
Synclonal
Synclonal
Synclonal
NA
NA
Synclonal/
Caryonidal
NA
NA
Synclonal
NA
No
No
NA
NA
NA
NA
No
INC
Yes
Yes
NA
Yes‡
NA
INC
NA
NA
INC
Cho (1971)
Nakata (1969)
Nakata (1969)
Esposito, Ricci & Nobili (1976)
Siegel (1956)
Webb & Francis (1969); Miyake
(1996)
Downs (1959)
Ammermann (1982)
Colgin-Bukovsan
(1976); Miyake (1996)
*Some individuals of these species are able to undergo autogamy, whereas others are not.
†Asexual species are denoted as having zero mating types.
‡This designation is tentative.
§Webb & Francis (1969) identify two mating types in this species, but allow that there may be more that have yet to be
identified.
¶These traits vary between syngens.
**Earlier studies suggest that this species may secrete pheromones (Dini & Nyberg, 1993).
INC, available data are inconclusive; NA, data not available for this trait.
Thus, based on the species for which we have data,
pheromone secretion does not appear to be a driving
factor in the evolution of mating type number.
Autogamy is a process of nuclear rearrangement
(meiosis, fertilization, and differentiation) within a
single cell in the absence of nuclear exchange, unlike
conjugation that involves genetic exchange between
two cells of compatible mating types. Autogamy
can be induced in some ciliate species by the same
environmental cues that induce conjugation (Siegel,
1956). Siegel (1956), Sonneborn (1957), and Miyake
(1996) hypothesized that autogamy should be more
common in species with binary mating type systems
because autogamy may provide an alternative means
of nuclear rearrangement when a cell of compatible
mating type cannot be found. Indeed, we observe an
association between autogamy and binary systems
(Table 1). Species with binary mating type systems in
Blepharisma and the Paramecium aurelia complex
undergo autogamy, whereas species with multiple
mating types (e.g. Tetrahymena spp., Glaucoma spp.,
and some Euplotes species) tend to not be capable of
autogamy. Exceptions to this trend, however, highlight
the need for further research on additional phylogenetically independent taxa. For example, Paramecium
caudatum, Paramecium sonneborni, and Paramecium
multimicronucleatum have binary mating type systems and are not capable of autogamy, and some
individuals of Euplotes crassus, Euplotes minuta, and
Euplotes vannus, which have multiple mating types,
are capable of autogamy, whereas others are not.
If this pattern is robust to further analysis, it would
indicate that species that are not capable of autogamy
are more likely to evolve multiple mating types
to increase the probability of finding a compatible
mating partner, but that this does not occur in autogamous species. This suggests that the necessity of
periodic nuclear reorganization may be a more important factor than genetic exchange and recombination
in determining the number of mating types, and
perhaps frequency of sex, in ciliates. Studies of clonal
aging in ciliates confirm the need for periodic nuclear
reorganization (Bell, 1988).
MATING TYPE DETERMINANTS
At least three scenarios can be proposed to explain
how the molecular determinants of mating type have
evolved within a species and diverged between species
of ciliates. Support for each of these models can be
found within at least a few ciliate lineages.
In Blepharisma, the molecular determinants of different mating types within a species are nonhomologous and structurally unrelated, but the determinant
for each mating type is homologous between species.
All species of Blepharisma that have been examined
possess two mating types (Table 1) (Miyake, 1996).
One of the mating types is determined by a 20-kDa
protein that is homologous, but variable, between
species. The other is determined by a nonpeptide
tryptophan derivative that is identical between
species. This observation suggests independent origins
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
CILIATE MATING SYSTEM EVOLUTION
B.
193
C. campylum
G. chattoni
A.
T. hyperangularis
* P. bursaria
100/80/92
97/61/66
P. caudatum
T. cosmopolitanis
<50/<50/<50
T. pigmentosa
95/<50/<50
P. multimicronucleatum
88/<50/<50
P. octaurelia
99/65/80
T. patula
T. capricornis
T. americanis
100/98/100
T. hegewischi
100/97/99
T. australis
P. tetraurelia
T. bergeri
100/99/97
+
P. sonneborni
T. corlissi
100/87/78
T. borealis
P. jenningsi
86/<50/<50
T. rostrata
98/95/99
T. canadensis
P. undecaurelia
98/<50/<50
+
98/<50/<50
<50/<50/<50
100/96/97
T. mobilis
T. tropicalis
P. quadecaurelia
99/71/85
T. malaccensis
T. thermophila
100/61/58
P. novaurelia
100/99/94
91/51/<50
84/<50/<50
T. elliotti
83/66/68
P. tredecaurelia
T. vorax
T. pyriformis
90/<50/<50
P. sexaurelia
100/66/81
T. setosa
99/53/<50
P. primaurelia
97/92/91
Halteria grandinella
P. decaurelia
88/75/100
53/<50/<50
C.
P. pentaurelia
Oxytricha granulifera
78/53/60
P. biaurelia
Stylonychia pustulata
P. dodecaurelia
E. raikovi
100/99/100
P. triaurelia
+
E. aediculatus
P. septaurelia
100/99/100
E. octocarinatus
mode of inheritance
Synclonal
Cytoplasmic
Caryonidal
equivocal reconstruction
number of mating types
2
+
100/92/99
7
3
8
4
9
5
10
6
12
E. patella
84/69/97
E. minuta
99/91/100
E. vannus
100/93/100
E. crassus
Figure 3. Phylogenies of Paramecium (A) based on hsp70 gene sequence and, Tetrahymena (B), and Euplotes (C) based
on SSU rDNA (see Supporting information, Table S1). Analyses were performed as in Fig. 2. Topologies shown are from
maximum likelihood analysis with support at nodes indicated as Bayesian posterior probability/maximum likelihood
bootstrap/maximum parsimony bootstrap. +, 100% support with each method. Parsimony-based ancestral character state
reconstructions are shown for mode of inheritance in Paramecium and mating type number in Tetrahymena and Euplotes;
both traits are treated as discrete and unordered. *Paramecium bursaria is mapped as having two mating types, but
syngens of this species vary in mating type number.
of the two mating type pheromones followed by
diversification of pheromone orthologs associated with
speciation.
In Paramecium, it is not known whether the
molecular determinants of different mating types
within a species are homologous, but they are known
to be produced by a shared pathway, and the determinant for each mating type is homologous between
species. All species of Paramecium that have been
studied, except P. bursaria, have two mating types,
designated O and E. In the biosynthesis of these
molecules, O may serve as the precursor to E (Xu
et al., 2001). In addition, O and E are determined by
orthologous genes across species (Tsukii, 1988). Thus,
it appears that the determinant for mating type E
evolved as a derivative of that for mating type O, and
that each determinant has subsequently diversified
across species. Weak mating reactions observed
between closely-related species (Murakami & Haga,
1995; Przybos et al., 2007) confirm the homology of
mating determinants between species and suggest
recent speciation and ongoing diversification of
mating type determinants.
In Euplotes, determinants of different mating types
are allelic or homologous within and between species.
Euplotes tend to have large numbers of mating types
within a species (Fig. 3, Table 1). All of the mating
type pheromones are encoded by homologous genes
(Luporini et al., 2005). This suggests that all pheromones in this genus represent the products of muta-
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
194
S. S. PHADKE and R. A. ZUFALL
tions in a single gene inherited from the common
ancestor of the Euplotes clade. In this system, mutations in mating type genes could result in new compatible mating types, or incompatible mating types,
the latter of which could lead to reproductive isolation.
MATING TYPE INHERITANCE
In many ciliates, conjugation and development result
in a change of mating type for the ex-conjugant
progeny. Genetic, epigenetic, environmental, and stochastic factors regulate the inheritance and expression of mating type genes (Nanney, 1956; Preer, 1968;
Arslanyolu & Doerder, 2000; Xu et al., 2001). Based
on the biology of reproduction and development, there
are three possible patterns of inheritance (Fig. 1). In
synclonal inheritance, all progeny of a conjugating
pair possess identical mating types representing their
identical germline genome. This type of inheritance is
strictly Mendelian. In cytoplasmic inheritance, each
progeny cell expresses the mating type of its parental
exconjugant. As a result, two different mating types
are expressed in the progeny of a conjugating pair
despite their identical germline genomes. This type
of inheritance may be explained by the epigenetic
influence of the parental macronucleus on the development of the new macronucleus (Koizumi & Kobayashi, 1989) possibly by an RNA interference-like
mechanism (Garnier et al., 2004; Mochizuki & Gorovsky, 2004). In caryonidal inheritance, the determination of mating type in each progeny cell depends on
the independent differentiation of the newly-formed
somatic nucleus and depends on genetic, epigenetic,
and environmental factors.
INHERITANCE
SYSTEM IS LABILE IN
PARAMECIUM
Species in the genus Paramecium exhibit all three
types of inheritance mechanisms. Synclonal inheritance is found within a single early-diverging clade
containing P. bursaria, P. caudatum, and P. multimicronucleatum. Based on parsimony-based ancestral
character state reconstruction, we hypothesize that
cytoplasmic inheritance gave rise to caryonidal inheritance at least once, and perhaps as many as four times
(Fig. 3). Paramecium is the only clade known to have
any species that undergo cytoplasmic inheritance.
These results suggest that cytoplasmic inheritance
may be an intermediate and evolutionarily unstable
state between synclonal and caryonidal inheritance.
INHERITANCE SYSTEM IS CONSERVED
TETRAHYMENA AND EUPLOTES
IN
Species in the genus Tetrahymena exhibit either synclonal or caryonidal inheritance. However, in contrast
to Paramecium, there appear to have been very few
transitions in inheritance mode in this genus: a single
transition from synclonal to caryonidal inheritance
(Fig. 3). The transition to caryonidal inheritance in
this clade may be associated with an increase in
mating type number. The significance of this correlation remains unclear. Species that exhibit synclonal
inheritance also show a peck-order dominance relationship among mating type alleles (T. australis clade;
Dini & Nyberg, 1993), whereas mating type alleles in
T. thermophila, a species that shows caryonidal inheritance, are codominant. The change in dominance
relationships between alleles may parallel the transition in mode of inheritance in Tetrahymena. Analysis of
dominance relationships in other species in the T.
thermophila clade could reveal the trend. All species in
the genus Euplotes that have been studied exhibit
synclonal inheritance. It is possible that the conservation of mode of inheritance across various species of
Euplotes reflects the homology of mating type genes.
SUMMARY
Research on mating systems in ciliates continues to
provide insights into intercellular signal transduction,
gene regulation, ligand-receptor interactions, and epigenetic inheritance. By examining these molecular and
genetic studies in a comparative framework, we have
seen that this research also provides insight into an
outstanding problem in evolutionary biology: the evolution of sexual systems. By analysing various aspects
of mating systems in a phylogenetic context, we find
that mating type number, molecular determinants of
mating types, and mode of mating type inheritance are
all evolving rapidly across ciliates. However, each
of these aspects appears to be under different
selective constraints in different lineages. We demonstrate several intriguing correlations between various
aspects of mating systems; however, the genetic, epigenetic, and selective factors responsible for both the
rapid diversification and lineage-based constraints
remain unclear. These results emphasize the need for
further research on organisms with complex and variable mating systems. In particular, detailed analyses
of taxonomically diverse ciliates will help to elucidate
the evolutionary mechanisms responsible for the diversification of mating systems and give us deeper insight
into the processes driving the evolution of sex.
ACKNOWLEDGEMENTS
We are grateful for the insightful comments and critiques of Brian Mahon, as well as two anonymous
reviewers. This work was supported by grants from
NSF (MCB-0625272) and the Environmental Institute of Houston.
© 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 187–197
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