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Fig. 2. Evolution of pharyngeal behaviors in free-living soil nematodes. Pharyngeal
behaviors in four free-living soil nematode families and an outgroup species
(Teratocephalus lirellus) are illustrated in A–E, where pumping
is represented by open pharyngeal lumens and peristalsis is represented by
arrows. The corpus pumped in all four families and pumped independently in all
families except the Rhabditidae. The isthmus and terminal bulb (TB), however,
exhibited significant differences regarding where pumping/peristalsis
occurred, as well as how different parts were coupled in their contractions
(see Results for more detailed descriptions). Thin grey lines are drawn
between the corpus and isthmus/TB in each schematic to help visualize the
largely conserved corpus behaviors and the varied isthmus/TB behaviors. (A)
Rhabditidae family (including C. elegans): pumping occurred in the
corpus, anterior isthmus, and TB, whereas peristalsis occurred in the
posterior isthmus. Corpus, anterior isthmus, and TB pumping were coupled. (B)
Diplogasteridae family: pumping occurred only in the corpus, whereas
peristalsis occurred in the isthmus and TB. Isthmus and TB were coupled to
conduct peristalsis. The Diplogasteridae TB lacks the grinder
(Maggenti, 1981;
Zhang and Baldwin, 1999). (C)
Cephalobidae family: pumping occurred in the corpus and terminal bulb, whereas
peristalsis occurred in the entire isthmus. Isthmus peristalsis and TB pumping
were coupled. (D) Panagrolaimidae: pumping occurred in the corpus, posterior
isthmus and TB, whereas peristalsis occurred in the anterior isthmus (AI).
Corpus pumping, anterior isthmus peristalsis, and posterior isthmus/TB pumping
all occurred independently. (E) T. lirellus: pumping occurred in the
corpus and TB, whereas peristalsis occurred in the entire isthmus. Corpus
pumping, isthmus peristalsis, and TB pumping all occurred independently. (F) A
model of how pharyngeal behaviors evolved in the isthmus and terminal bulb of
free-living soil nematodes, together with the currently accepted phylogenetic
relationships between each family (Blaxter
et al., 1998; Felix et al.,
2000; Goldstein et al.,
1998). Spatial pattern refers to where pumping/peristalsis
occurred, whereas coupling pattern refers to how the different motions were
coupled. By comparison to the outgroup species, T. lirellus, the
spatial pattern in the Cephalobidae is ancestral, whereas the coupling pattern
in the Panagrolaimidae is ancestral. Open circles and squares on the
phylogenetic tree indicate that spatial and coupling patterns, respectively,
evolved from an ancestral pattern to a derived pattern in the subsequent
lineage. For example, the Rhabditidae lineage is marked with both a circle and
a square to denote that both its spatial and coupling patterns are derived.
AI, anterior isthmus; PI, posterior isthmus; TB, terminal bulb.
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