Feeding Dimorphism in a Mycophagous Nematode, Bursaphelenchus sinensis

Phenotypic plasticity has been widely reported in animals and can drive investment in new biological characters that engender ecological adaptability. The nematode family Diplogastridae, especially Pristionchus pacificus with its dramatic stomatal (feeding) dimorphism, has become an important model system to analyze the evolutionary and developmental aspects of polyphenism. However, this plasticity has not been confirmed in other nematode groups. In the present study, we experimentally examined the feeding dimorphism of a fungal feeding free-living nematode, Bursaphelenchus sinensis. In a laboratory culturing experiment, the nematode expressed dimorphism, i.e., a small proportion of the population manifested as a predatory form. This form only occurred in females and was not clearly influenced by the presence of potential prey species. In addition, the ratio of the predatory form to the mycophagous form varied among different fungal food species grown in monoculture on different culture media. The predatory form of B. sinensis was typologically similar to the monomorphic (specialized) predators belonging to the same family. However, some essential morphological characters were slightly different from the specialized predators, and their behaviours were clearly disparate, suggesting that predation in B. sinensis is derived from a different phylogenetic origin than that of the specialized predators.


Supplementary Text S1. Morphological description of Bursaphelenchus sinensis
The typological characters of the mycophagous form adults have already been described in previous studies. Therefore, some important characters to compare with those of the predatory form and close relatives, anterior part of body and tail region, are described here. The morphological characters were photomicrographed in Fig. 1 and drawn in Supplementary Fig.   S1.

Mycophagous form adult
Cuticle thin, annulated, lateral field with two lines. Head distinctly offset from body, separated by a clear constriction, lip region in lateral view squarish rounded, ca twice as broad as high. Stylet with narrow lumen comprising a short cone ca one-third or slightly more of total stylet length and a shaft with small but clear basal swellings. Procorpus cylindrical, ca three stylet lengths (= metacorpal lengths) long, ending in well-developed metacorpus. Metacorpal valve clearly observed, present at middle of, or slightly posterior to, centre of metacorpus (median bulb). Dorsal pharyngeal gland orifice opening into lumen of metacorpus mid-way between anterior end of metacorpal valve and anterior end of metacorpus. Pharyngo-intestinal junction slightly posterior to metacorpus. Dorsal pharyngeal glands narrow, i.e., less than onethird of corresponding body diam., ca five stylet (metacarpal) lengths, overlapping intestine dorsally. Nerve ring surrounding pharyngeal glands and intestine slightly posterior to pharyngointestinal junction. Secretory-excretory pore opening ventrally around the level of nerve ring.
Hemizonid at ca one metacorpal length posterior to metacorpus, unclear in live material but distinct in permanently mounted material.

Predatory form adult
Predatory form was found only in females in the present study. Cuticle thin to moderate, coarsely annulated, lateral field with two lines. Head distinctly offset from body, separated by a clear constriction, lip region in lateral view crown-like, i.e., square-shaped subventral and dorsal sectors protrude laterally, ca three times as broad as high. Stylet with wide lumen comprising a conus which is a little less than half of total stylet length and a shaft with small but clear basal swellings. The stylet lumen opens ventrally like the tip of an injection needle. Procorpus cylindrical, less than 1.5 stylet lengths (= metacorpal lengths) long, ending in well-developed metacorpus. Metacorpal valve clearly observed, prominent, present at middle of metacorpus (median bulb). Dorsal pharyngeal gland orifice opening into lumen of metacorpus mid-way between anterior end of metacorpal valve and anterior end of metacorpus. Pharyngo-intestinal junction immediately posterior to metacorpus. Dorsal pharyngeal glands wide and short, i.e., more than half of corresponding body diam., less than three stylet (metacorpal) lengths, overlapping intestine dorsally,. Nerve ring surrounding pharyngeal glands and intestine slightly posterior to pharyngo-intestinal junction. Secretory-excretory pore opening ventrally around the level of nerve ring. Hemizonid at ca 1.5 metacorpal lengths posterior to metacorpus, unclear in live material but distinct in permanent mounted material.

Male
Spicules, paired. Capitulum (rostrum + condyles) not well developed, forming ventrally inclined roundish rectangle where condyles and rostrum are not clearly distinctive. Dorsal limb of spicule (lamina) prominent, with almost straight anterior two-thirds and smoothly ventrally curved posterior one-third; posterior end with roundish-squared terminus without cucullus.
Ventral limb (calomus) inconspicuous, forming triangular cuticle membrane. Gubernaculum or apophysis absent. Bursal flap present, narrow, tape-like, inconspicuous, starting from level of second paired genital papillae (P3) or posterior, not clearly observed from lateral aspect. Seven genital papillae: one ventral papilla (P1) and three ventral subventral paired papillae (P2, P3, P5) present: precloacal P1 papilliform, ventral, slightly anterior to cloacal opening (co); adcloacal P2 papilliform, on subventral body; P3 located mid-way between co and tail tip (tip of bursal flap) or slightly anterior; gland opening-like P5 on ventral side slightly posterior to mid-way between P3 and tail tip, possessing internal connection (unclear secretory duct-like structure).  Data for three replicates are shown in %, and the mean is given in parentheses.

Supplementary
3) Aphelenchoides sp. is seemingly a parthenogenetic species (no male was found). Values within a column with the same characters are not significantly different (P = 0.05).