Hatching in nematodes

M.Sc. IV Semester Nematology Behaviour and Physiology of nematodes

Inside the egg In general, at least three requirements must be fulfilled for efficient hatch: 1. the physiological mechanism of the nematode must be operating efficiently; 2. the nematode must be highly mobile; and 3. it has to possess the anatomical requirements for locomotion and penetration out of the egg

Movements inside the egg enables the nematode to reach a pitch of physiological efficiency at the time it hatches. The stylet, is necessary for the escape out of the cyst. The stylet bearing juvenile makes a line of overlapping punctures with its stylet, forming a slit through it escapes later. ………..

Hatching largely studied in plant parasites with special reference to cyst nematodes

Egg shell permeability According to Perry et al. (1982), PRD (Potato root diffusate) increases the permeability of the lipoprotein membrane of the eggshell, allowing the egg to take up water. Egg shell permeability is affected by a change in calcium ion concentration, which changes the structural lipids ( lipid globules) PRD changes egg shell permeability and subsequent water uptake by the juvenile leads to hatching Water uptake by the juvenile also increases oxygen consumption inside the egg. The changes in egg shell permeability occur during the first 24 h after exposure to PRD, but it takes 3 days for juvenile hatching (eclosion) to take place

Behavioural response Behavioural sequence leading to hatching widespread exploration when the juvenile becomes fully mobile inside the egg, local exploration when only the head of the juvenile is moving while the lip region is pressed to the eggshell, and the stage when the juvenile cuts its way out of the eggshell with the stylet After hatching the juvenile takes up more water before it emerges from the cyst

The sequence of events during hatching in Ascaris 1. Changes in eggshell permeability 2. Activation of the larva 3. Eclosion

As a result of permeability change, there is absorption of water and escape of trehalose from the perivitelline fluid into external medium. In cyst nematodes the potato root diffusate causes change in permability and the leakage of trehalose in mediated by Ca++ ions in G. rostochiensis and by Zn++ in H. glycines In Ascaris the permeability change is Na+ mediated process with Na+ forming a complex with lipid layer

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Initiation of hatching Process

Sequence of events

Larval movements leading to hatching

Response to hatching agents The response to hatching has been shown to differ between the two PCN species. After hatch stimulation, a series of physical and metabolic changes occur in the unhatched juvenile. The hatching response of PCN cysts to PRD is generally fast. After very short exposure (5 min) the PCN juveniles are triggered to start substantial hatching from the cyst. Tests in vitro on G. pallida show that it has a much slower initial rate of hatch compared with G. rostochiensis

Hatching factors There are three major factors that have an effect on nematode cyst hatching, namely host root diffusate, physical conditions before and during juvenile emergence, and hereditary preconditions of the cyst (Manduric, 2004). The hatching mechanism of PCN has been shown to be stimulated by PRD, temperature, oxygen availability, soil type and soil water content.

Potato root diffusate Peak production is for a short period in the plant’s life that stimulates juveniles to hatch. Maximum activity of PRD is reached two weeks after planting Two classes of chemicals have been found in PRD, hatch inhibitors and hatch stimulants. PRD contains at least four to six different hatching agents (chemical stimulants). The inhibitors are produced at an earlier stage than the stimulants, protecting the plant during its most sensitive stage of growth PRD is highly mobile in soil and induces hatching up to 80 cm Hatching compound Solanoeclepin A in potatoes is hatching agent for G. rostochiensis A hatching factor, glycinoeclepin A, for H. glycines eggs isolated from the roots of kidney beans.

Temperature Temperature is an important factor in the hatching of PCN. PCN requires a relative stable temperature regime to hatch: the optimum hatching temperature in the field is 13.4 C but substantial emergence can be observed at 10 C. The optimum temperature for G. rostochiensis hatching in vitro is 20 C. G. pallida is adapted to lower temperatures (16 C) At the optimum temperature the fastest growth, reproduction, etc. occur. At temperatures below and above the optimum, the rates are slower or death occurs.

Soil and water status Soil structure and soil water status are important factors for maximum hatching of PCN, RKN and all other soil-borne nematodes. Coarse-textured soils favour hatching and when the water content is at field capacity the maximum hatching occurs. Drought and water-logging inhibit the hatching of plant parasitic nematodes Cultivar A fully susceptible plant allows the nematode to multiply free on the roots while the partially susceptible plant allows less multiplication. Thus hatching occurs at a greater rate in host plant For the resistant cultivars or non host plant hatching is lowered

Diffusates from host roots increased hatching of H. glycines in one study while those from nonhosts had no effect Hatching activity was not found in another study of root diffusates from soybean plants. However, diffusates from the roots of plants such as potato stimulated hatching of G. pallida eggs, whereas diffusates from mustard roots, a nonhost, inhibited hatching. Other chemicals Zinc chloride stimulated more hatching of eggs of the soybean cyst nematode Heterodera glycines than did calcium Zinc chloride induced little hatching of the G. rostochiensis eggs.

Signals between plants and organisms Most signals between organisms and their host plant/s are based on phytochemicals from the host plant, but the organism can also release chemicals to the host plant. Biotic factors Soil micro-organisms living in the rhizosphere affect hatching. According to Ryan and Jones (2004), who tested 70 rhizobacterial isolates, nine of the isolates had a significant effect on hatching. Six of these affected the hatching of G. pallida and three affected G. rostochiensis. The PCN attained optimum hatching, 43-100% more hatching, when treated with a potato isolate that had been soaked in PRD for 48 h, while no effect was found when treated with isolate without PRD.

Other related events

•Delayed hatching

•Embryonic arrest in M. naasi

•Diapause

•Often diapause in G. rostochiensis is initiated by signals from the plant during the growing season (Hominick et al. 1985).

•Nematodes develop inside the female and are arrested at J2, after moulting from J1.

•This development has been observed in eggs outside the female (cyst) and the arrest point remains in the J2

DIAPAUSE

Suggested reading (online resources) doi: 10.1017/S0031182012001874 DOI: 10.1016/0169-4758(89)90299-8 https://doi.org/10.1017/S0031182000085413 https://www.youtube.com/watch?v=i5scxw_S240

With context to C. elegans https://www.wormatlas.org/embryo/introduction/mainframe.htm https://www.youtube.com/watch?v=M2ApXHhYbaw http://www.wormbook.org/db/misc/search.cgi?search_html=on;search_preprints=on;query=hatching%20rate