Description of Caenorhabditis sinica sp. n. (Nematoda: Rhabditidae), a nematode species used in comparative biology for C. elegans

Abstract

We re-isolated in China a relative of the nematode model Caenorhabditis elegans that was previously referred to informally as C. sp. 5. In spite of its importance for comparative biology, C. sp. 5 has remained morphologically uncharacterized. Therefore, we now provide detailed description of morphology and anatomy, assigning the name of Caenorhabditis sinica sp. n. to this nematode that is found frequently in China. C. sinica sp. n. belongs to the Elegans group in the genus Caenorhabditis, being phylogenetically close to C. briggsae although differing in reproductive mode. The gonochoristic C. sinica sp. n. displays two significantly larger distal parts of uteri filled with sperms in the female/hermaphroditic gonad than does the androdioecious C. briggsae. The new species can be differentiated morphologically from all known Caenorhabditis species within the Elegans group by presenting a uniquely shaped, three-pointed hook structure on the male precloacal lip. The lateral field of C. sinica sp. n. is marked by three ridges that are flanked by two additional incisures, sometimes appearing as five ridges in total. This study ends the prolonged period of the 'undescribed' anonymity for C. sinica sp. n. since its discovery and use in comparative biological research. Significant and crossing-direction dependent hybrid incompatibilities in F1 and F2 crossing progeny make C. sinica sp. n. an excellent model for studies of population and speciation genetics. The abundance of nematode species lacking detailed taxonomic characterization deserves renewed attention to address the species description gap for this important yet morphologically 'difficult' group of animals.

Figure 1. Line drawings of Caenorhabditis sinica sp. n.

A: Overall anatomy of female (left) and male (right)[St: stoma, Fl: flap, Phar: pharynx, Ne: nerve ring, Va: valvular apparatus, Ha: haustrulum, Ex: excretory pore, Ca: cardia (the pharyngo-intestinal valve), In: intestine, Ger: “germigen” containing a well-developed central rachis surrounded by a layer of germ cells, OoE: elongated oocytes, Ov: oviduct, dUs: the distal part of anterior/posterior uterus filled with sperms, Ut: uterus, Em: embryos carried by the uteri, Vu: vulva, An: anus, Phas: phasmid, Te: testis, Bu: bursa, GP: genital papillae, Gu: gubernaculum, Sp: spicule, pch: precloacal hook]; B: Morphology of lateral field (3 ridges flanked by two additional incisures); C: Anterior region of female; D: Lateral view of male caudal region; E: Ventral view of male caudal region with bursa and genital papillae. (Scale bars: A = 200 µm; B = 10 µm; C = 150 µm; D, E = 50 µm).

Figure 2. Micrographs of Caenorhabditis sinica sp. n.

(A–I) A: Entire view of female (left) and male (right); B: Pharynx (v: valvular apparatus; h: haustrulum); C: Female tail; D: Didelphic-amphidelphic female gonad (Ov: oviduct, Vu: vulva, dUs: the distal part of anterior/posterior uterus filled with sperms, arrow shows the posterior direction); E: Spermatheca formed by the oviduct (spm) and the distal part of anterior uterus (dUs); F: Anterior end of female; G: Lateral view of male caudal regions; H, I: Ventral view of male caudal regions showing bursa or its nine pairs of genital papillae (GP1, GP5, GP7: terminated on the dorsal side by a conspicuous sensillum tip). J: The distal part of anterior uterus of a wild C. briggsae strain ZZY0405 (indicated by arrow). (Scale bars: A = 800 µm; B, C, D = 200 µm; E = 100 µm; F = 10 µm; G, H, I, J = 50 µm).

Figure 3. Scanning electron micrographs of Caenorhabditis sinica sp. n.

A, B: Anterior regions of female (A) and male (1–6: six labial sensilla; a: amphid; CS: cephalic sensilla, which are present only in males); C: Lateral field of female; D: Lateral field of male; E, F, G, H: Male caudal regions enveloped by a closed bursa and its nine pairs of genital papillae (pch: precloacal hook; Lpch: the two lateral points of the precloacal hook; pg: posterior part of gubernaculum; GP1, GP5, GP7: showing their dorsal tips being of typical papilliform sensilla).

Figure 4. Hybrid viability between our five wild isolates and Caenorhabditis sinica sp. n. reference strains.

A: percentage of male and embryonic lethality for C. sinica sp. n. (JU1201) itself and its F1 crossing progeny with our five wild isolates. Shown are data derived from one crossing direction, i.e., crossing between JU1201 males and ZZY females. Significant difference (p<0.05, Student's t test) between JU1201 and the crossing progeny is indicated with “*”. Numbers of adult progeny counted are indicated in the bottom (in parenthesis). Error bars denote standard deviations. B and C: percentage of embryonic lethality of F1 and F2 hybrid progeny between C. sinica sp. n. (JU727) and the five wild isolates respectively. Crossings were performed in both forward and reverse directions as indicated (See Materials and Methods for details). F1 hybrid males and females derived from the crossing in either direction were mated and F2 embryonic lethality was scored for their progeny. Significant difference (p<0.05, Student's t test) in the percentage of embryonic lethality between JU727 and the crossing progeny is marked with “*”. Numbers of total embryos counted are indicated above each bar.