Early embryonic development of the central nervous system in the Australian crayfish and the Marbled crayfish (Marmorkrebs)

Abstract

This study sets out to provide a systematic analysis of the development of the primordial central nervous system (CNS) in embryos of two decapod crustaceans, the Australian crayfish Cherax destructor (Malacostraca, Decapoda, Astacida) and the parthenogenetic Marbled crayfish (Marmorkrebs, Malacostraca, Decapoda, Astacida) by histochemical labelling with phalloidin, a general marker for actin. One goal of our study was to examine the neurogenesis in these two organisms with a higher temporal resolution than previous studies did. The second goal was to explore if there are any developmental differences between the parthenogenetic Marmorkrebs and the sexually reproducing Australian crayfish. We found that in the embryos of both species the sequence of neurogenetic events and the architecture of the embryonic CNS are identical. The naupliar neuromeres proto-, deuto-, tritocerebrum, and the mandibular neuromeres emerge simultaneously. After this "naupliar brain" has formed, there is a certain time lag before the maxilla one primordium develops and before the more caudal neuromeres follow sequentially in the characteristic anterior-posterior gradient. Because the malacostracan egg-nauplius represents a re-capitulation of a conserved ancestral information, which is expressed during development, we speculate that the naupliar brain also conserves an ancestral piece of information on how the brain architecture of an early crustacean or even arthropod ancestor may have looked like. Furthermore, we compare the architecture of the embryonic crayfish CNS to that of the brain and thoracic neuromeres in insects and discuss the similarities and differences that we found against an evolutionary background.

Engrailed-like immunoreactivity in the embryonic ventral nerve cord of the Marbled Crayfish (Marmorkrebs)

“…Phalloidin histochemistry to label actin reveals the axonal architecture of the embryonic brain at this stage ( Fig. 5b; compare Vilpoux et al 2006). A primordial scaffold of the brain is present in the protocerebral (optic anlagen), deutocerebral (antenna 1), and tritocerebral (antenna 2) neuromeres as well as the mandible segment (representing the ''naupliar brain''; Vilpoux et al 2006).…”

“…5b; compare Vilpoux et al 2006). A primordial scaffold of the brain is present in the protocerebral (optic anlagen), deutocerebral (antenna 1), and tritocerebral (antenna 2) neuromeres as well as the mandible segment (representing the ''naupliar brain''; Vilpoux et al 2006). However, there is no evidence for axonal tracts stretching further caudally from the mandible segment at E38% (Fig.…”

“…The embryonic nervous system was labelled with phallotoxins conjugated to a fluorescent dye (Vilpoux et al 2006). Briefly, dissected embryos were fixed in 4% paraformaldehyde in 0.1 M phosphate buffer (PB, pH 7.4) for 4 h at room temperature.…”

“…Yet, at this stage, an average (n = 3) of only 11 labelled cells have emerged in the neuroectoderm of thoracomere one. In E39% embryos, the mandible and the maxilla one Vilpoux et al 2006) neuromeres contain around 36 En-li? cells (n = 3), while in the maxilla two neuromere we charted an average (n = 3) of 27 and in the thoracomere one an average of 18 En-li?…”

“…By apomictic propagation, the offspring conserve the original genetic information and, thus, are identical clones of the mother (Martin et al 2007). Due to its high fertility the Marbled Crayfish is an excellent model to explore aspects of crustacean embryology and neurobiology (Seitz et al 2005;Alwes and Scholtz 2006;Vilpoux et al 2006;Polanska et al 2007;Sintoni et al 2007;Rieger and Harzsch 2008).…”