Adaptation to temperature in entomopathogenic nematodes

Jagdale, Ganpati Baburao (1997) Adaptation to temperature in entomopathogenic nematodes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The effects of recycling over a two-year period at temperatures from 10 - 25 °C were studied in four strains of entomopathogenic nematodes: Steinernema carpocapsae All strain, Steinernema feltiae NF strain, Steinernema feltiae Umeå strain and Steinernema riobravis TX strain. The main objectives of this investigation were to study the capacity for, and mechanisms involved in. thermal adaptation of a native (NF strain) boreal steinernematid nematode and to compare it with strains of entomopathogenic nematodes originating from another boreal region (Sweden; Umeå strain), temperate zone (All strain) and subtropical zone (TX strain). -- On the basis of morphometry and restriction fragment length polymorphism in ribosomal DNA spacer, a nematode that was isolated from soil near St. John's, Newfoundland, Canada, was designated as a new strain, NF, of S. feltiae. To determine whether taxonomic separations of entomopathogenic nematodes could be accomplished on the basis of physiological properties other than DNA profiles, cellulose acetate electrophoresis was used to separate isozymes of eight enzymes in infective juveniles of five strains of Steinernema. These strains comprised the four listed above, together with S. feltiae L1C strain, isolated 14 years previously from Newfoundland. Based on comparisons of the relative electrophoretic mobilities (μ) of the isozymes, it was concluded that the five isolates could be distinguished. This finding suggests that cellulose acetate electrophoresis could be an important taxonomic tool for the identification of isolates of steinernematids, provided that a combination of enzymes is used. -- The capacities for high temperatures and freezing tolerance were determined and found to be affected by the temperatures at which recycling was carried out. In all the isolates, the UT₅₀, the temperature at which 50% of the infective juveniles died, increased with increase in recycling temperature. Freezing tolerance, as measured by times at which 50% of the infective juveniles were killed (Lt₅₀) by a -5 °C regime, was reduced at higher recycling temperatures. Boreally adapted S. feltiae was more cold tolerant than S. carpocapsae and S riobravis, but its capacity to withstand freezing was reduced by recycling at warmer temperatures. The greatest capacity for heat tolerance was observed in S. riobravis, while S. carpocapsae displayed an intermediate degree of tolerance to high temperatures. -- Infectivity was found to be modified by the recycling temperatures. This was examined by measuring LD₅₀ values in G. mellonella larvae at bioassay temperatures from 5 to 25 °C. Only the infective juveniles of S. feltiae that had been recycled at 10 °C infected and killed the insects at a 5 °C bioassay temperature, whereas S. carpocapsae and S. riobravis were infective at 10 °C only when the recycling temperatures were ≤ 20 °C. The infectivity of the two strains of S. feltiae at 10 or 15 °C was compromised by propagating them at warmer temperatures (20 - 25 °C). The S. feltiae Umeå strain displayed an impaired capacity to infect hosts at warm temperatures (20- 25 °C) when recycled at colder (≤15 °C) temperatures. -- The pattern of isozymes was modified according to the recycling temperature regime. In all four isolates, isozyme banding patterns of malate dehydrogenase (MDH), marmoset-phosphate isomerase (MPl) and phosphoglucomutase (PGM) were affected by recycling or storage temperature. The two strains of S. feltiae synthesized additional isozymes of MPI and MDH or PGM in response to cold temperatures, while S. carpocapsae All strain synthesized three isozymes of MDH in response to warm temperatures. Modifications of isozyme synthesis may constitute a temperature adaptation mechanism for these nematodes. -- In all four isolates, the kinetic properties of glucose-6-phosphate dehydrogenase and hexokinase were affected by the recycling temperature. At each assay temperature (5 - 35 °C), the maximum specific activity of both the enzymes was greater in the nematodes that had been recycled at lower temperatures than in those reared at higher temperatures. In three enzyme - nematode isolate combinations, the lowest Km values occurred in nematodes that had been recycled at the lower temperatures. -- Using gas liquid chromatography, it was determined that the composition of fatty acids in total lipids and phospholipids changed adaptively with recycling temperatures. In all four isolates, the unsaturation indices of total lipids and phospholipids were increased as temperature decreased. This was due to an increase in polyunsaturated fatty acids a with concomitant decline in the proportion of saturated fatty acids, especially palmitic (16:0) and/or stearic (18:0) acids. The increase in polyunsaturated fatty acids at reduced temperatures was attributed to significantly greater percentages of linoleic acid (18:2) in total lipids and phospholipids. In all except S. riobravis, this was augmented by increased proportions of eicosapenic acid (20:5w3) at 5 °C. -- Together, these results suggest that the various recycling temperatures influenced the capacities of the entomopathogenic nematodes for temperature tolerance and infectivity at an organismal level. At the physiological level, the synthesis of isozymes, enzyme kinetics, and composition of fatty acids in lipids were modified by the recycling temperatures in an apparently adaptive fashion. The degree to which the various physiological changes provide an underlying basis for those at the organismal level is discussed.

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