Cellular pattern formation, establishment of polarity and segregation of colored cytoplasm in embryos of the nematode Romanomermis culicivorax.

We have begun to analyze the early embryogenesis of Romanomermis culicivorax, an insect-parasitic nematode phylogenetically distant to Caenorhabditis elegans. Development of R. culicivorax differs from C. elegans in many aspects including establishment of polarity, formation of embryonic axes and the pattern of asymmetric cleavages. Here, a polarity reversal in the germline takes place already in P(1) rather than P(2), the dorsal-ventral axis appears to be inverted and gut fate is derived from the AB rather than from the EMS blastomere. So far unique for nematodes is the presence of colored cytoplasm and its segregation into one specific founder cell. Normal development observed after experimentally induced abnormal partitioning of pigment indicates that it is not involved in cell specification. Another typical feature is prominent midbodies (MB). We investigated the role of the MB region in the establishment of asymmetry. After its irradiation the potential for unequal cleavage in somatic and germline cells as well as differential distribution of pigment are lost. This indicates a crucial involvement of this region for spindle orientation, positioning, and cytoplasmic segregation. A scenario is sketched suggesting why and how during evolution the observed differences between R. culicivorax and C. elegans may have evolved.

[Fecundity and embryonal development of Romanomermis culicivorax]. / Fecundidad y desarrollo embrionario de Romanomermis culicivorax.

Romanomermis culicivorax (Ross and Smith, 1976) eggs completed the late preparasite curling phase of their embryo development in an average time of 11 days and at a temperature of 25 +/- 2 degrees C. It was also observed that female R culicivorax specimens laid 312 eggs in 13 days, when male and female specimens were placed in a substrate of distilled water with a pH = 4.5.

Effect of temperature and host-parasite ratio on sex differentiation of Romanomermis iyengari (Welch), a mermithid parasite of mosquitoes.

The effect of temperature and host-parasite ratio on the percentage infection and sex differentiation of R. iyengari was studied. Significant differences were observed in the percentage infection due to different host-parasite ratios and temperatures. At 25 degrees and 30 degrees C, the host parasite ratio of 1:3 resulted in 86-92% infection of Culex quinquefasciatus larvae. At 20 degrees and 35 degrees C, a higher host-parasite ratio was required to get this level of infection. More number of post-parasites per mosquito larva emerged at 20 degrees (1.5-5.8) and 25 degrees C (1.9-6.3) than at 30 degrees (1.5-3.9) and 35 degrees C (1.6-3.6). More than 50% of the post-parasites were females at 20 degrees and 25 degrees, 30 degrees and 35 degrees C at 1:1-1:10, 1:1-1:4 and 1:1-1:3 host-parasite ratios, respectively.