Hello everyone, welcome to this chapter dedicated on solitary ascidians.
Solitary and colonial ascidians share a common life cycle based on sexual reproduction.
While colonial ascidians can also propagate by asexual reproduction, as you will see
in the next chapter, solitary ascidians exclusively propagate by sexual reproduction.
It takes a few months, depending on the species, from the release of the gametes
- spermatozoids and oocytes - and the subsequent fertilisation
to obtain a sexually mature adult.
The life cycle can be managed in the lab for a few species, such as Ciona intestinalis,
Ciona savignyi, Ciona robusta, or Phallusia mammillata, and allowed the emergence
of some lab strains and also some genetic manipulation as you will see in the next chapter.
Typical solitary ascidians are characterized by a bi-phasic life cycle
composed by swimming larvae - the pelagic phase - and sessile adults - the benthic phase.
Since the larva swims freely from a few hours to a few days depending the species
the benthic phase represents almost the entire life cycle, while the pelagic phase
is transitory and allows the dispersion of individual animals.
Adults are usually hermaphrodites, producing both sperm and oocytes.
Gamete production is seasonal and synchronisation among individuals
of gametes release is induced by light.
Cross-fertilisation occurs rapidly in seawater after spawning
and is followed by rapid embryogenesis.
Ascidian embryogenesis is a rapid process since for example at 18°C it takes 18 hours
from fertilisation to a fully-formed swimming larva in C. intestinalis or C. robusta.
Following fertilisation, a series of rapid cell divisions occurs
approximately every 20 minutes, this characterising the cleavage stage.
The animal-vegetal axis is established during oogenesis, while the anterior-posterior
axis arises perpendicularly to the animal-vegetal axis after fertilisation.
This is notably due to the asymetric location of maternal determinants.
This asymetric partitioning of maternal determinants is called ooplasmic segregation.
In this picture you can see the partitioning of the three main maternal determinants:
muscle, endoderm, and epidermis.
The developmental fates of blastomeres are restricted very early during embryogenesis.
As you can see in this picture, at the onset of gastrulation, meaning the 110-cell stage,
most of the blastomeres are already fated to give rise to a single type of tissue.
This is represented by different colours. You can recognise in violet, epidermis,
in green, muscle, and in blue, endoderm, corresponding to the lineage of the blastomere
that inherited the maternal determinant previously partitioned during ooplastic segregation.
One remarkable feature of ascidian embryonic development is that they
develop stereotypically, with few cells and invariant cell lineages
so that each blastomere can be named and found at the same position in all embryos.
Cell lineages of ascidian embryos are well-established and are invariant
between individuals. In this picture you can see the lineage that is common to all ascidians
with few differences in some species. The lineage and the nomenclature
of each blastomere correspond to its position regarding the A-P axis
with A for Anterior blastomere and B for Posterior blastomere and the A-V axis,
with the uppercase letter for vegetal blastomeres and the lower case for animal.
Another remarkable property of ascidian embryogenesis is that stereotypical
embryogenesis and cell lineages are invariant between embryos within and
between species, even for species with a long phylogenetic distance.
For example, C. robusta and P. mamillata split more than 200,000,000 years ago
and look very similar during early embryogenesis in their geometry.
In other words, during early embryogenesis, all ascidians look the same.
At the end of embryogenesis, the fully- formed larva starts to move its tail,
coupled with the apoptosis of the surrounding cells, contribute to the hatching.
From the hatching, the lavae adopt a pleagic behaviour by swimming and
dispersing in the environment.
The ascidian larvae present a morphology divided in two parts: the anterior trunk,
and the posterior tail. The larva is usually composed of low number of cells, 2,600
in the case of Ciona robusta here on this picture.
This typical anatomy is common to the solitary ascidians' larva represented by
this diagram. The totality of the larval body is surrounded by a tunic which is
composed by a cellulose derivative: tunicine.
The epidermis, under the tunic, covers the entire animal body.
Two internal tissues are distributed along the entire antero-posterior axis:
the CNS, characterised by a dorsal neural tube, and neurons in adhesive papillae,
essential for metamorphosis. The second tissue is the endoderm.
The tail,
the notochord plays a role of support and the muscle allows the swimming movement.
At the tip of the tail, the primordial germ cells, which will
give rise to the gonads after metamorphosis.
The settlement is the first step of metamorphosis. This is mediated
through the adhesive papillae located at the most anteroir extremity of the larva.
This is done on substrates both natural and artificial that present a bacterial film.
This metamorphosis will be detailed later in the research part of this module.
Ascidian metamorphosis was first described by Cloney in 1982, leading to the
identification of 10 successive steps shared by almost all ascidian species
with few variations. But, the most dramatic event of the metamorphosis shared by all
the tailed species is the regression of the tail, characterised by the disappearance
of almost all tail tissue.
Apoptosis appears to be the driving force of the tail regression and affects almost all
of the cells that compose the tail, which are the tunic, the endoderm, the epidermis,
the notochord, and the muscle cells, but also the central nervous system cells.
However, there are two exceptions: the endodermal strand cells and the primordial
germ cells that move towards the trunk prior or during the tail regression.
In this picture with a double labelling - in red, the WGA to visualise the larva
and in green, the TUNEL assay that detects the massive apoptotic cells
that occur during the tail regression.
Metamorphosis in ascidians results in a dramatic modification of their body plan,
transforming them in a few hours from a swimming larva to a sessile juvenile
and after a few months of growing to a sexually mature adult.
Classically, juvenile growth timing depends on food availability and temperature,
Solitary ascidian adults are characterised by a bag-shaped morphology in which the
foot is fixated on the substrate and two siphons at the opposite end.
The larger siphon is the inhalant one, which allows the entry of seawater
which moves into the pharynx, facilitating respiration and nutrient filtration.
At the base of the pharynx is the oesophagus driving food particles to the stomach.
Near the stomach, the heart manages the circulation through the animal
thanks to a few vessels carrying blood cells.
Around the stomach and the heart, the gonads that produce both sperm and oocytes
which accumulate in two separate gonaducts alongside the exhalant siphon,
the smallest one, where they are released during spawning.
Between the two siphons, the nerve ganglion from which the innervation is made towards
the other organs, among them, the muscles distributed all over the animal
participating in body shape maintenance, pharynx contraction to control the water
flow and open across the two siphons
To conclude this chapter on solitary ascidians, I encourage you to visualise
the different animations that you will find on the website page,
like the ascidian life cycles, the colonial but also the solitary, and to test your
knowledge, lets go to the different quizzes that you will find also on the Moodle page.