| Abstract |
The development of multicellular organisms relies on the specification of different cell types and their
sorting and assembly into distinct tissues and organs. Cell sorting refers to the segregation of a mixed
population of non-identical cells into distinct domains as well as the active maintenance of the
segregated compartments. The process of cell sorting appears to be conserved between vertebrates and
invertebrates, and was postulated to occur in embryonic, postnatal, and adult tissues. Cell sorting and
assembly is controlled by various molecular interactions among the constituent cells, but is ultimately
governed by differences between the physical properties of the specific cell types.
In metazoans, the elongation of the anteroposterior axis is a conserved developmental process and an
exemplary developmental event relying on tissue convergence along the dorsal-ventral (DV) axis and
extension along the anterior-posterior axis. In Drosophila, a common model organism in developmental
biology, genetic studies have revealed patterns of transcription factor expression that provide spatial and
temporal cues that induce oriented cell movements and promote germband extension. Nonetheless, the
cellular mechanisms by which patterned transcriptional inputs orchestrate cell polarity and behaviour
events had long been elusive. Zallen and colleagues have recently identified leucine-rich repeat and
other transmembrane receptors that are expressed in transverse stripes along the head-to-tail axis and
direct planar polarity and polarized cell rearrangements during germband convergent extension in
Drosophila. Thus, tissue-level patterns of expression of transmembrane receptors provide spatial signals
that link positional information from the AP patterning system to the essential cell behaviors that drive
convergent extension
To determine how leucine-rich repeat and other transmembrane orthologs pattern the germband during
the AP axis formation in other arthropods, I have chosen to study the amphipod crustacean Parhyale
hawaiensis. Parhyale appears to be an exemplary model system that allows for single-cell analysis of the
morphological process of germband formation. Similarly to other amphipod crustaceans, the ectoderm
of Parhyale initially condenses from an unorganized population of cells into an organized grid of
transverse rows of cells (perpendicular to AP axis) and longitudinal columns of cells (parallel to AP axis).
Each transverse row of cells corresponds to one parasegment (ParaSegment Precursor Row - PSPR)
termed ‘abcd’, will undergo two rounds of division along the AP axis. The first division generatesC two
rows termed 'ab' and 'cd', which in turn become rows 'a','b','c', and 'd' following the second division.
4-row parasegments are easily discerned by the expression of the segment polarity gene engrailed in the
anterior ‘a’ row. The progressive addition of new PSPRs at the posterior end of the germband and their
stereotypic longitudinal divisions contribute to Parhyale germband elongation.
In this dissertation, I give a brief overview of the principles underlying cell sorting in development, and
describe exemplary developmental processes in which cell sorting is observed. Additionally, I identify the
orthologs of 4 Drosophila melanogaster genes, tartan, capricious, Tanascin-major and Tenascin-accessory
in Parhyale and investigate their phylogeny across the phylum of Arthropoda. Subsequently, I use labeled
RNA probes to examine their expression during the embryonic stages of germband formation in
Parhyale.
Our results indicate that Parhyale has four tartan/capricious orthologs and three ten-m/-a orthologs in
total. All of them are expressed at the forming germand (embryonic stages 14 - 17) and exhibit almost
identical expression patterns. The phylogenetic analysis and survey of the Parhyale genome support the
theory that these genes are not the result of recent gene duplication events in Parhyale, raising
questions about the specific roles in development for each gene. Functional analysis of these genes using
CRISPR to knock out single genes and also groups of similar genes will reveal further information about
their possible roles in the organization of the Parhyale germband.
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Role of leucine-rich repeats and teneurin transmembrane proteins in pancrustacean germband organization
