CONTENT - International Society of Zoological Sciences

S11 ICZ2008 - Abstracts
S12 - Transitions from clonal to sexual reproduction: key variations of a key process
The function of sperm donating species to the evolution and
persistence of unisexual salamanders (caudata: ambystomatidae)
Jim Bogart
Department of Integrative Biology, University of Guelph, N1G 2W1,
Guelph, CANADA
Unisexual salamanders in the genus Ambystoma are common
around the Great Lakes region in eastern North America. Based on
mitochondrial DNA sequences, they originated from a single
hybridization event that involved a female that shared a common
ancestor with Kentucky A. barbouri 2.4 to 3.9 million years ago.
Individual unisexual salamanders reproduce by stealing sperm from
donors of normally bisexual species so their reproductive mode is
described as kleptogenesis. As many as 22 different diploid and
polyploid unisexual biotypes are known and they all possess at least
one A. laterale genome. One or more other nuclear genomes have
been derived from sperm donors that may involve males of five
distinct species. Sperm may serve only to stimulate egg
development (gynogenesis) but can be incorporated to replace a
nuclear genome or to elevate ploidy. Genome replacement is
considered to be an essential evolutionary strategy to rectify meiotic
problems that arise from recombinations and translocations among
gynogenetic offspring and to introduce novel genetic combinations
that have a selective advantage. Molecular tools such as
microsatellite DNA loci, expressed sequence tags (EST), and
genomic in situ hybridization (GISH) can be used to identify the
genomic constitution of individuals and to test hypotheses pertaining
to the role of sperm donors in the co-evolution of the disparate
cytoplasmic and nuclear components in unisexual salamanders.
Bogart, J. 2003. Genetics and systematics of hybrid species. Pp.
109-134, In D. M. Sever (edt.), Reproductive Biology and Phylogeny
of Urodela. M/s Science Inc., Enfield NH.
Bogart, J. P., K. Bi, J. Fu, D.W.A. Noble, and J. Niedzwieki. 2007.
Unisexual salamanders (genus Ambystoma) present a new
reproductive mode for eukaryotes. Genome 50: 119-136.
Bi, K., J. P. Bogart, and J. Fu. 2008. Genealogical relationships of
unisexual salamanders of the genus Ambystoma inferred from
intergenomic exchanges and 45S rDNA cytotypes. Chromosome
Research 16: 275-289.
Sex in asexuals: how occasional sex determines the fate of
parthenogenetic populations
Thomas D'Souza
Auf der Morgenstelle 26, 72076, Tuebingen, Germany
Sex is expensive compared to asexual reproduction. Nevertheless
sex is more widespread among multi-cellular organisms. Many
theories have been developed to solve this so-called paradox of sex
and to clarify why sex is advantageous. Particularly when combining
these theories, the benefits of sex seem to outweigh the costs.
Consequently, asexuals are usually considered as evolutionary
dead-ends. This ra! ises the question whether asexual systems are
always truly clonal or whether they have cryptic forms of sexuality
that enhances their viability. This is important as, at least
theoretically, a limited amount of sexuality is sufficient to compensate
for the long-term costs of clonality. Here, the advantages of
occasional sex will be demonstrated using the planarian flatworm
Schmidtea polychroa as an example. Parthenogenetic S. polychroa
are simultaneous hermaphrodites, which require sperm from a
partner in order to trigger embryo development (pseudogamy). As
parthenogens are hermaphroditic, they produce fertile sperm and do
not rely on sexual sperm donors. Parthenogenesis in S. polychroa is
not always strictly clonal, but sometimes contains occasional, sexual
process. This mixed reproduction mode may combine the benefits of
sex and asex and increases the survival of parthenogenetic
populations in two different ways. Firstly, occasional sex promotes
the investment in male reproductive organs of parthenogens, which
is essential for rare sex among parthenogens.
- 44 -
Secondly, occasional sex in parthenogenetic S. polychroa leads to
an increase in fitness variance at the local population level, which in
turn increases mean fitness, which can be interpreted as a sign!
ature of a more effective natural selection.
Sexual reproduction in triploid forms of planarian Dugesia
ryukyuensis
Kazuya Kobayashi 1 , Hirotsugu Ishizu 1 , Motonori Hoshi 2 and Midori
Matsumoto 1
1
Biosciences and Informatics, Keio University, Hiyoshi 3-14-1,
Kouhoku-ku, Yokohama, 223-8522, Japan.
2
University of the Air, Wakaba 2-11, Mihama-ku, Chiba, 261-8586,
Japan.
Triploidy has generally been considered to be an evolutionary dead
end due to problems of chromosomal pairing and segregation during
meiosis. Thus, the formation of tetraploids and diploids from triploid
types is a rare phenomenon. In the planarian Dugesia ryukyuensis,
both fissiparous and oviparous triploids occur in nature. It is believed
that oviparous triploids reproduce via pseudogamy (spermdependent
parthenogenesis). We have successfully induced
experimental sexualization of fissiparous triploids in D. ryukyuensis.
Following sexualization, the triploids develop hermaphroditic gonads
and other reproductive organs and begin reproducing by copulation
instead of fission. In the present study, we demonstrated that
inbreeding of the triploid planarian D. ryukyuensis resulted in both
diploid and triploid offspring in nature. In the triploids of D.
ryukyuensis, chiasmata between homologous chromosomes were
observed in both female and male germ lines. This result suggests
that both diploid and triploid offspring of this species are produced
bisexually by zygotic fusion between sperm and eggs. Hence, this
phenomenon may be a novel mechanism in planarian for escaping
the triploid state.
Males in a thelytokous strain of tardigrade
Atsushi C. Suzuki
Department of Biology, Keio University, Hiyoshi, Yokohama 223-
8521, Japan
Tardigrades are generally gonochoristic and propagate by sexual
reproduction. A number of tardigrade species exhibit secondary
sexual characters, with males in some Eutardigrada being
distinguished by modified claws. Many moss-dwelling tardigrades,
however, have thelytokous mode of parthenogenesis. One of such
tardigrade is Milnesium tardigradum. Males in this species with
modified claws are usually rare and many populations appear to
have only parthenogenetic reproduction. The presence of males in
small numbers, being greatly different from the sex ratio of 1:1, had
not been considered as heterogony so far. Instead, it was interpreted
to be the result of mixed populations that have parthenogenetic and
amphimictic reproduction. Here the author reports that males have
emerged at a very low frequency in a thelytokous strain of Milnesium
cf. tardigradum, which has been maintained since 2000. Some
individuals of this strain had modified claws characteristic of males
on the first pair of legs. A small testis filling with many spermatozoa
was observed in one of such specimen. Although copulation has not
been observed yet, an interesting activity of a male against a female
appeared to be a sexual behaviour. The frequency of the emergence
of males in the cultured strain is so low that any environmental factor
that may generate males has not been determined yet. It is unknown
if these males could actually function as males in reproduction.
However, they might show some possibility of genetic exchange
among the clonal populations.