Light Sheet-based Fluorescence Microscopy of Living or Fixed and Stained Tribolium castaneum Embryos

Summary

Imaging the morphogenesis of insect embryos with light sheet-based fluorescence microscopy has become state of the art. This protocol outlines and compares three mounting techniques appropriate for Tribolium castaneum embryos, introduces two novel custom-made transgenic lines well suited for live imaging, discusses essential quality controls and indicates current experimental limitations.

Abstract

The red flour beetle Tribolium castaneum has become an important insect model organism in developmental genetics and evolutionary developmental biology. The observation of Tribolium embryos with light sheet-based fluorescence microscopy has multiple advantages over conventional widefield and confocal fluorescence microscopy. Due to the unique properties of a light sheet-based microscope, three dimensional images of living specimens can be recorded with high signal-to-noise ratios and significantly reduced photo-bleaching as well as photo-toxicity along multiple directions over periods that last several days. With more than four years of methodological development and a continuous increase of data, the time seems appropriate to establish standard operating procedures for the usage of light sheet technology in the Tribolium community as well as in the insect community at large. This protocol describes three mounting techniques suitable for different purposes, presents two novel custom-made transgenic Tribolium lines appropriate for long-term live imaging, suggests five fluorescent dyes to label intracellular structures of fixed embryos and provides information on data post-processing for the timely evaluation of the recorded data. Representative results concentrate on long-term live imaging, optical sectioning and the observation of the same embryo along multiple directions. The respective datasets are provided as a downloadable resource. Finally, the protocol discusses quality controls for live imaging assays, current limitations and the applicability of the outlined procedures to other insect species.

This protocol is primarily intended for developmental biologists who seek imaging solutions that outperform standard laboratory equipment. It promotes the continuous attempt to close the gap between the technically orientated laboratories/communities, which develop and refine microscopy methodologically, and the life science laboratories/communities, which require 'plug-and-play' solutions to technical challenges. Furthermore, it supports an axiomatic approach that moves the biological questions into the center of attention.

Introduction

The red flour beetle Tribolium castaneum, which belongs to the large family of darkling beetles (Tenebrionidae), has a long history within the agricultural and life sciences and is the second best studied model insect model organism after the fruit fly Drosophila melanogaster. During the last four decades, it became a powerful and popular insect model organism in developmental genetics, in evolutionary developmental biology and, during the last twenty years, in embryonic morphogenesis for a variety of reasons:

Drosophila and Tribolium both belong to the Holometabola, but diverged approximately 300 million....

Protocol

1. Husbandry of Tribolium Cultures

NOTE: Standard conditions are defined as an incubation temperature of 25 °C and 70% relative humidity in a 12 h bright/12 h dark cycle. For more information on Tribolium husbandry, respective guidelines are available⁶⁴. This protocol requires two different flour-based media, which can be prepared in kilogram quantities and stored for several months.

1. Prior to working with flour and inactive dry yeast, store the unopened packages at -20 °C for 24 h and then let them warm up to room temperature. This procedure eliminates potential pathogens.
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Results

This protocol describes an experimental framework for fluorescence imaging of living or fixed and stained Tribolium embryos with LSFM. Due to the low levels of photo-bleaching and photo-toxicity, a direct consequence of its optical sectioning capability, LSFM is particularly well suited for long-term live imaging.

The novel AGOC{ATub'H2B-mEmerald} #1 transgenic line expresses a histone2B-mEmerald fusion protein under co.......

Discussion

Quality control

In live imaging assays, the preparation and recording procedure must be non-invasive, i.e. neither the mechanical and chemical handling (collection, dechorionation, mounting onto the sample holder) nor the integrated energy load during the observation should affect the viability of the specimen. For studies that characterize WT development, it is recommended to only use data from experiments in which the embryo survives the recording process, is retrie.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
full grain wheat flour	Demeter e.V.	1.13E+08	US: whole wheat flour, UK: whole meal flour
405 fine wheat flour	Demeter e.V.	1.13E+08	US: pastry flour, UK: soft flour
inactive dry yeast	Flystuff / Genesee Scientific	62-106
Phosphate-buffered Saine (PBS), pH 7.4	Thermo Fisher Scientific	10010-023
sodium hypochlorite, ~12% active Cl	Sigma Aldrich	425044-250ML	Caution: sodium hypochlorite is corrosive
low-melting agarose	Carl Roth	6351.2
6-well plate	Orange Scientific	4430500
24-well plate	Orange Scientific	4430300
glass capillaries, internal Ø 0.46 mm	Brand GmbH + Co KG	7087 09
SYTOX Green	Thermo Fisher Scientific	57020	Staining solution preparation is explained in Table 2
YOYO-1 Iodide	Thermo Fisher Scientific	Y3601	Staining solution preparation is explained in Table 2
BOBO-3 Iodide	Thermo Fisher Scientific	B3586	Staining solution preparation is explained in Table 2
Alexa Fluor 488 Phalloidin	Thermo Fisher Scientific	A12379	Staining solution preparation is explained in Table 2
Alexa Fluor 546 Phalloidin	Thermo Fisher Scientific	A22283	Staining solution preparation is explained in Table 2
sieve, 800 µm mesh size	VWR International	200.025.222-051
sieve, 710 µm mesh size	VWR International	200.025.222-050	for growth medium preparation (step 1.1)
sieve, 300 µm mesh size	VWR International	200.025.222-040
sieve, 250 µm mesh size	VWR International	200.025.222-038	for egg laying medium preparation (step 1.2)
glass dish, Ø 100 mm × 20 mm	Sigma Aldrich	CLS70165102
cell strainer, 100 µm mesh size	BD Biosciences	352360
paint brush, head Ø 2 mm	VWR International	149-2121
syringe, 1.0 mL	B. Braun Medical AG	9166017V
scintillation vials	Sigma Aldrich	M1152-1000EA
paraformaldehyde (PFA)	Sigma Aldrich	158127	Caution: paraformaldehyde is toxic and corrosive
n-heptane ≥99%	Carl Roth	8654.1	Caution: n-heptane is flammable and toxic
Triton X-100	Sigma Aldrich	X100-100ML	Caution: Trition X-100 is corrosive