Cortical microtubule nucleation can organise the cytoskeleton of $Drosophila$ oocytes to define the anteroposterior axis

TL;DR

This study presents a theoretical model demonstrating that cortical microtubule nucleation organizes the cytoskeleton in Drosophila oocytes, crucial for establishing the anterior-posterior axis and mRNA localization.

Contribution

The paper introduces the first comprehensive 3D model of non-centrosomal microtubule organization in Drosophila oocytes, emphasizing cortical nucleation's role in polarity.

Findings

A simple cortical MT nucleation gradient reproduces observed MT distribution.

Cytoplasmic flows are not essential for mRNA localization.

Modulating cortical nucleation causes polarity phenotypes.

Abstract

Many cells contain non-centrosomal arrays of microtubules (MT), but the assembly, organisation and function of these arrays are poorly understood. We present the first theoretical model for the non-centrosomal MT cytoskeleton in $Drosophila$ oocytes, in which $bicoid$ and $oskar$ mRNAs become localised to establish the anterior-posterior body axis. Constrained by experimental measurements, the model shows that a simple gradient of cortical MT nucleation is sufficient to reproduce the observed MT distribution, cytoplasmic flow patterns and localisation of $oskar$ and naive $bicoid$ mRNAs. Our simulations exclude a major role for cytoplasmic flows in localisation and reveal an organisation of the MT cytoskeleton that is more ordered than previously thought. Furthermore, modulating cortical MT nucleation induces a bifurcation in cytoskeletal organisation that accounts for the phenotypes of polarity mutants. Thus, our three-dimensional model explains many features of the MT network and highlights the importance of differential cortical MT nucleation for axis formation.