article has not abstract
“To us as embyrologists and men the formation of an embryo has
appeared to be everything, the history of the germ cells a secondary item of
no particular moment. Nature, on the other hand, reverses the relative
importance of the two, setting the germ-cells in the place of honour, as
linking the remote past with the distant future.”
In 1902, the vertebrate embryologist James Beard wrote these words in his monograph
on germ cells in the skate Raja batis
. Over 110 years
later, germ line specification and development are indeed major areas of
investigation in the fields of developmental biology and evolution. August
Weismann's description of the germ line as containing “unalterable
accessory idioplasm”  may sound suspiciously mythical to modern readers.
Nevertheless, we now know that in some animals, a special cytoplasm containing
conserved gene products is indeed transmitted from oocyte to embryonic germ cells,
and again to oocytes in the next generation. This “germ plasm” is
necessary and sufficient for germ cell formation, and its molecular basis is best
understood in the fruit fly Drosophila melanogaster. Germ plasm in
some form has been described in oocytes and embryos of most “higher
insects” (Holometabola: e.g., flies, wasps, and butterflies) as well as in
other animals such as frogs and fish. However, in “lower insects”
(Hemimetabola: e.g., grasshoppers, mayflies, and cockroaches) and in most other
animals, nothing like germ plasm or inherited germ line determinants have been
reported. In mice, the best studied example of such cases, inductive signals from
specific somatic cells cause neighbouring cells to adopt germ cell fate. Comparative
analyses suggest that most animals may use inductive signaling rather than germ
plasm to specify germ cells, including animals branching close to the base of the
animal tree (e.g., sponges and cnidarians). This has led to the hypothesis that the
ancestral mechanism for animal germ cell specification may have been based on
inductive signaling, meaning that germ plasm-driven mechanisms would have evolved
independently several times in animal radiation . How such a novel mechanism
could have evolved remains unclear. In a recent paper , Jeremy Lynch and colleagues
provide evidence that a critical component of germ plasm in insects is more ancient
than previously thought, and that the driving force for this novel mechanism was the
evolution of a novel gene.
The Lone Ranger
Insects fall into two major groups: the Holometabola (“higher insects”)
show indirect development through a pupa or chrysalis stage, while the Hemimetabola
(“lower insects”) develop directly, without metamorphosis. All orders of
holometabolous insects contain species where, as observed for
Drosophila, germ cells are exclusively derived from a small
number of cells that form at the posterior pole of the embryo shortly after
fertilization . These “pole cells” have also been described
in some beetles and hymenopterans (bees, ants, and wasps), including the wasp
Nasonia vitripennis. However, other members of the same insect
orders, including the beetle Tribolium castaneum and the honeybee
Apis mellifera, do not form pole cells , , and the molecular mechanism
used to specify germ cells in these insects is presumed to be inductive. Similarly,
hemimetabolous insects such as cockroaches and grasshoppers do not have pole cells
Pole cells acquire their germ cell fate by inheriting cytoplasmic determinants, or
germ plasm. In Drosophila, when germ plasm is removed or destroyed,
pole cells cannot form and the animal is sterile , . Conversely, transplanting germ
plasm to ectopic locations causes ectopic germ cells to form , . It turns out that there is
only one gene described in D. melanogaster whose products are also
necessary and sufficient for germ cell formation: oskar. Uncovered
in genetic screens for maternal effect mutations , its transcript and protein
are localized to the posterior cytoplasm of the oocyte and early embryo. When
overexpressed in ectopic locations, oskar induces ectopic germ
plasm and germ cell formation , .
Surprisingly, unlike many other genes with indispensable roles in development,
oskar is not a widely conserved gene: it proved absent from the
first non-fly insect genomes sequenced, and has no clear homologue in any other
animal. Although the orthologue from another fly (Drosophila
immigrans) can substitute functionally for D. melanogaster
, that from an
equally distantly related fly (Drosophila virilis) cannot . This suggests
that the fruit fly strategy for assembling germ plasm evolved very recently, in the
lineage leading to the Diptera (flies and mosquitoes), but is not widely applicable
in other insects. Because oskar encodes for a novel protein with
unknown function, its evolutionary origins have remained an even deeper mystery.
Lynch and colleagues now pull back the curtain on the evolution of
oskar, revealing that it evolved in higher insects long before
the appearance of fruit flies.
The Search for Family
The wasp Nasonia vitripennis belongs to the Hymenoptera (ants, bees,
and wasps), which are likely to be the most basally branching order of
holometabolous insects . Nasonia is an attractive model to study
the evolution of germ plasm, because it is easy to culture in the lab, has a
sequenced genome, robust protocols for gene expression and functional analysis, and
derives its germ line from pole cells. Examining the sequenced genome of this wasp,
Lynch and colleagues found an oskar orthologue
(Nv-osk) using a relaxed and modified BLAST strategy. They
found that part of the protein has similarities to a family of proteins called
tudor-domain-containing (Tdrd) proteins, some of whose members have documented roles
in germ cell development in other animals. This suggests that oskar
may have evolved by duplication and subsequent divergence of a gene that already had
a germ cell role. As in Drosophila, Nv-osk is
localized to the posterior of the oocyte and early embryos, and knocking down
Nv-osk by RNAi results in disrupted germ plasm and no pole
cells. However, it also results in a range of somatic patterning defects, suggesting
that unlike fly oskar, Nv-osk may play complex
roles outside of the germline as well. The authors then investigated the upstream
regulation of Nv-osk by examining the roles of two genes that
regulate oskar translation in flies, bruno and
Hrp48. Knockdown of the wasp homologues of these translational
regulators resulted in abnormally localized Nv-osk transcripts,
suggesting that some aspects of oskar regulation may also have
Nasonia's phylogenetic position means it is possible that any
characters that it shares with Drosophila, including
oskar, were present in the last common ancestor of all
holometabolous insects. However, several holometabolous insects lack pole cells,
including Nasonia's close relative the honeybee (Apis
mellifera), whose genome also lacks an oskar
homologue. This suggests that oskar or germ plasm may have been
secondarily lost in some higher insect lineages. To determine whether the
oskar/germ plasm/pole cells relationship was conserved in other
hymenopterans, Lynch and colleagues searched for, and found, an
oskar homologue in the ant Messor pergandei.
Mp-osk transcripts localize to the posterior
of oocytes and embryos, and the embryos of these ants have pole cells.
Back to Our Roots
The authors' choice of model organism and use of multiple dipteran
oskar orthologues as queries to their wasp genome allowed them
to find an oskar homologue in a lineage further removed from
Drosophila than had been previously suspected. This work has
not simply added another sequence to our meager list of oskar
homologues; it also predicts that the origins of this gene could be at least 300
million years old (the estimated time of divergence of Hymenoptera from Diptera
further prediction from this work is that higher insects as diverse as beetles,
moths, and fleas should have oskar homologues as well. Given the
apparent rapid evolutionary rate of this gene and the absence of genome sequences
for most of these insects, these homologues may be challenging to identify, but
their study could yield further important insights into the evolution of germ line
specification in these animals.
What about other animals, like Xenopus, Caenorhabditis
elegans, and zebrafish, which have maternally inherited germ line
determinants but no oskar homologues? A zebrafish gene called
bucky ball has been reported to have
oskar-like genetic properties, but has no detectable homology to
the work on Nv-osk sheds light on this problem as well. We know
that the genetic networks regulating germ cell development on the one hand, and
subcellular localization mechanisms including translational control on the other
hand, are ancient metazoan mechanisms , . This suggests that the advent
of novel oskar-like molecules capable of interacting with both
networks could have facilitated the evolution of novel modes of specifying germ
cells. Future work could take advantage of this prediction based on known modularity
of mechanisms, by searching for germ plasm nucleators on the basis of molecular
properties, rather than traditional homology.
Finally, finding more oskar homologues may give us insight into
mechanisms of neofunctionalization and the evolution of novel protein functions.
While one region of oskar may have its origin in a duplicated Tdrd
gene, the C terminus of Oskar has the greatest (but still weak) similarity not to
animal gene domains, but to SGNH/GDSL hydrolases of bacterial species! Lynch and
colleagues discuss the possibility that horizontal gene transfer from bacterial
endosymbionts could have led to the fusion of domains from animal and insect genes.
Although speculative, this is not completely outside the realm of possibility, as
there is a widespread association between insects and endosymbiotic bacteria, which
have often been found to colonize the germline of their hosts.
Understanding the evolutionary processes that created this puzzling gene will
undoubtedly be more difficult than elucidating its mechanism of action in extant
animals. Nevertheless, the effort will be worth the reward, as thinking broadly
about the origins of genetic innovation can help us understand not just how new
genes can arise, but also how these new genes can lead to the evolution of novel
The germ cells. I. Raja batis.
Zoologische Jahrbucher Abteilung fur Anatomie und Ontogenie der
The germ-plasm: a theory of heredity.
Walter Scott, Ltd
Mechanisms of germ cell specification across the metazoans:
epigenesis and preformation.
The phylogenetic origin of oskar coincided with
the origin of maternally provisioned germ plasm and pole cells at the base
of the Holometabola.
The origin of the germ cells in Drosophila
The embryology of the honey bee.
Princeton University Press
Tribolium embryogenesis: a SEM study of cell
shapes and movements from blastoderm to serosal closure.
Dev Genes Evol
The embryology of Blatta germanica and
Studies on the embryology of the African migratory locust,
Locusta migratoria migratoides Reiche and Frm. II.
Philos Trans R Soc Lond B Biol Sci
Action de l'ultra-violet sur le pôle germinal dans
l'oef de Drosophila melanogaster.
Revue Suisse de Zoologie
Manipulation of the pole plasm of Drosophila
Acta Embryologiae et Morphologiae Experimentalis
Transplantation of posterior polar plasm in
Drosophila. Induction of germ cells at the anterior
pole of the egg.
Proc Natl Acad Sci U S A
The autonomous function of germ plasm in a somatic region of the
Exp Cell Res
Abdominal Segmentation, pole cell formation, and embryonic
polarity require the localized activity of oskar, a
maternal gene in Drosophila.
Induction of germ cell formation by
Overexpression of oskar directs ectopic
activation of nanos and presumptive pole cell formation in
Oskar controls morphology of polar granules and nuclear bodies in
Drosophila virilis oskar transgenes direct body
patterning but not pole cell formation or maintenance of mRNA localization
in D. melanogaster.
Phylogenomic analysis reveals bees and wasps (Hymenoptera) at the
base of the radiation of Holometabolous insects.
Evolution of the insects.
Cambridge University Press
Bucky ball organizes germ plasm assembly in
The molecular machinery of germ line
Mol Reprod Dev
Molecular mechanisms of translational control.
Nat Rev Mol Cell Biol