Unexpected Variation in Neuroanatomy among Diverse Nematode Species

Abstract

Nematodes are considered excellent models for understanding fundamental aspects of neuron function. However, nematodes are less frequently used as models for examining the evolution of nervous systems. While the habitats and behaviors of nematodes are diverse, the neuroanatomy of nematodes is often considered highly conserved. A small number of nematode species greatly influences our understanding of nematode neurobiology. The free-living species Caenorhabditis elegans and, to a lesser extent, the mammalian gastrointestinal parasite Ascaris suum are, historically, the primary sources of knowledge regarding nematode neurobiology. Despite differences in size and habitat, C. elegans and A. suum share a surprisingly similar neuroanatomy. Here, we examined species across several clades in the phylum Nematoda and show that there is a surprising degree of neuroanatomical variation both within and among nematode clades when compared to C. elegans and Ascaris. We found variation in the numbers of neurons in the ventral nerve cord and dye-filling pattern of sensory neurons. For example, we found that Pristionchus pacificus, a bacterial feeding species used for comparative developmental research had 20% fewer ventral cord neurons compared to C. elegans. Steinernema carpocapsae, an insect-parasitic nematode capable of jumping behavior, had 40% more ventral cord neurons than C. elegans. Interestingly, the non-jumping congeneric nematode, S. glaseri showed an identical number of ventral cord neurons as S. carpocapsae. There was also variability in the timing of neurodevelopment of the ventral cord with two of five species that hatch as second-stage juveniles showing delayed neurodevelopment. We also found unexpected variation in the dye-filling of sensory neurons among examined species. Again, sensory neuron dye-filling pattern did not strictly correlate with phylogeny. Our results demonstrate that variation in nematode neuroanatomy is more prevalent than previously assumed and recommend this diverse phylum for future "evo-devo-neuro" studies.

Keywords: Heterodera; Heterorhabditis; Meloidogyne; Pratylenchus; amphid; heterochrony; invertebrate; phasmid.

FIGURE 1

Phylogeny of nematode species discussed in this study. The phylum Nematoda is currently divided into 12 clades as discussed in van Megen et al. (2009). Branch lengths do not represent distance.

FIGURE 2

DAPI staining of wild-type Caenorhabditis elegans (Clade 9). The ventral nerve cord (VNC) consists of a line of motor neurons extending along the ventral midline from the retrovesicular ganglion (RVG) to the pre-anal ganglion (PAG). (A) Ventral view of DAPI stained animal. (B) Region surrounding end of RVG and anterior portion of VNC (arrow). (C) Part of the VNC showing neuronal (arrowheads) and hypodermal (arrow) nuclei. (D) Division between PAG and VNC (arrow). Inset scale bars = 5 μm.

FIGURE 3

The VNC of nematodes in Clades 8–12 is highly variable. Fluorescent micrographs of individual nematode species fixed in formaldehyde and exposed to DAPI followed by imaging under fluorescent light. Species examined include: (A) Pristionchus pacificus hermaphrodite (Clade 9). (B) Heterorhabditis bacteriophora infective juvenile (Clade 9). (C) Heterorhabditis megidis infective juvenile (Clade 9). (D) Steinernema carpocapsae infective juvenile (Clade 10). (E) Steinernema glaseri infective juvenile (Clade 10). (F) Acrobeles sp. adult female (Clade 11). (G) Aphelenchus avenae J3 (Clade 12). (H) Meloidogyne hapla J2 (Clade 12). (I) Pratylenchus penetrans adult female (Clade 12). (J) Heterodera glycines J2 (Clade 12). Scale bar = 20 μm.

FIGURE 4

Aphelenchus avenae undergoes a post-hatch increase in the number of VNC neurons during J2. The number of VNC nuclei was examined with DIC microscopy in synchronized A. avenae nematodes at various time-points following hatch. The molt from J2 to J3 occurs at approximately 36 h after hatching. Each data point represents an individual animal.

FIGURE 5

DiI-filling is highly variable among nematodes. Live nematodes were exposed to DiI for 2 h followed by repeated washes in water or buffer and then imaged with fluorescent microscopy. (A) Left Anterior of the Clade 9 nematode Caenorhabditis elegans dauer, arrowhead indicates amphid neurons. (A) Right Posterior of a Caenorhabditis elegans dauer, two pairs of phasmid neurons are shown. (B) Left Anterior of the Clade 10 nematode Steinernema carpocapsae female, arrow indicates inner labial neurons. (B) Right Posterior of a S. carpocapsae male, arrow indicates phasmid neurons and arrowheads indicate unidentified neurons. (C) Left Anterior of the Clade 11 nematode Acrobeles sp. female, arrow indicates amphid neurons and arrowhead indicates inner labial neurons. (C) Right Posterior of an Acrobeles sp. female, two pairs of phasmid neurons are shown. (D) Left Anterior of the Clade 12 nematode Aphelenchus avenae, pentagon indicates amphid neurons, arrow indicates cephalic neurons, and arrow head indicates inner labial neurons. (D) Right Posterior of an Aphelenchus avenae female, two pairs of phasmid neurons are shown. (E) Anterior of the Clade 12 nematode Pratylenchus penetrans female (ventral view), one pair of amphid neurons in the anterior of the nematode is shown. Scale bar = 10 μm for all images.