This paper (i) reviews temperature/development rate relationships in plants and poikilothermic invertebrates, (ii) argues that the relationship is often linear over much of the range up to the thermal optimum (T o ) and provides a possible mechanism, (iii) provides evidence of a trade-off between the base temperature (T b ) and the thermal constant (DD) that enables each species to adapt to its thermal environment, and (iv) indicates some of the practical and ecological implications. Where a linear relationship has been characterised it is possible to estimate the base temperature for development (T b , expressed in °C) and the thermal constant for development (DD, the reciprocal of the temperature coefficient (a), expressed in degree [°C] days accumulated above T b ). A possible basis for the linear relationship between rate and temperature is proposed based on the Arrhenius and Sharpe-Schoolfield equations involving activation enthalpy and progressive inactivation of the reactant molecules at both low and high temperatures. Knowledge of T b and DD enables rates of development of organisms/ processes to be calculated and compared at any given temperature between T b and T o . An analysis of published results for differentiation processes (differentiation = a change of state) in species of insects, Collembola, spiders, nematodes and plants showed that T b tended to vary with the temperature of the niche to which the organism is adapted, and that there was a trade-off between T b and DD. Tropical species had higher values of T b than temperate and DD decreased as T b increased (and vice versa). This conferred a competitive advantage on each species in the thermal environment to which it was adapted. The decrease in DD tended to be relatively greater than the increase in T b, further favouring a high T b in tropical species. A mechanism for the trade-off is suggested whereby DD and T b were shown to be correlated (P < 0.01) with the activation enthalpy (H A ) of an assumed, rate-limiting enzyme. Thermal time can also be applied to processes involving growth (= an increase in dry weight) when the DD requirement for development to maturity is the sum of the requirements for differentiation and growth. Rates of both differentiation and growth can vary greatly between species, depending upon the niche they inhabit, and the implications of such differences for resource requirements are considered. In insects and nematodes, but not in annual plants, development is usually coupled to growth. Consequently, when resources are inadequate, mature size in these animals varies less than in plants. Thermal time is shown to provide insight into the life strategies of species within their communities and to have practical implications.

Most apomictic root-knot nematodes (RKN; Meloidogyne spp.) have host ranges that encompass the majority of flowering plants, and M. incognita is possibly the world's most damaging crop pathogen. The ancestors, age, and origins of the polyphagous RKN are obscure, but there is increasing evidence that M. incognita, M. javanica, and M. arenaria are closely related, heterogeneous species with a recent, hybrid (reticulate) origin. If so, they must owe much of their current worldwide distributions to spread by agriculture. Host resistance appears to be generally durable in the field, but laboratory studies suggest that apomixis does not prevent evolution in response to selection by a parasitic bacterium (Pasteuria penetrans) and host resistance. Maintaining general fitness may be the evolutionary priority for most populations of polyphagous RKN, and a wide host range, important in the field but not in the laboratory, may be conserved by apomixis. Several factors may help confer a wide host range, including suppression of host resistance, perhaps as a consequence of the strength of the induced susceptible response. Resistance genes effective against RKN appear not to have resulted from coevolution. Rates of juvenile invasion and/or development are low in many wild and some crop plants, with the result that they are both poor hosts and sustain less damage. Overall, it is suggested that greater coordination, particularly of fundamental research, is required.

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The conditions to which larvae of H. rostochiensis were exposed in host-plant roots determined the sex ratio of the adults. Apparently only second-stage larvae with giant cells of a sufficient size and nutrient content can become females. As the density of invading larvae increases progressively fewer find a site capable of containing such a giant cell and an increasing proportion become males. Most larvae that invade lateral roots become males.

This essay considers biotrophic cyst and root-knot nematodes in relation to their biology, host-parasite interactions and molecular genetics. These nematodes have to face the biological consequences of the physical constraints imposed by the soil environment in which they live while their hosts inhabit both above and below ground environments. The two groups of nematodes appear to have adopted radically different solutions to these problems with the result that one group is a host specialist and reproduces sexually while the other has an enormous host range and reproduces by mitotic parthenogenesis. We consider what is known about the modes of parasitism used by these nematodes and how it relates to their host range, including the surprising finding that parasitism genes in both nematode groups have been recruited from bacteria. The nuclear and mitochondrial genomes of these two nematode groups are very different and we consider how these findings relate to the biology of the organisms.