Developmental decisions and processes can be controlled transcriptionally. The free-living nematode Caenorhabditis elegans
makes a developmental decision between different larval fates. This decision is based on the 'suitability' of the environment for growth and reproduction. Under 'favourable' conditions, second stage larvae (L2) develop via
two larval stages (L3, L4) into reproductive adults [1
]. However, under 'unfavourable' conditions, L2s form a developmentally arrested L3 stage, the so-called dauer larva. Dauer larvae are environmentally resistant, have a specialised metabolism and are comparatively long-lived [2
]. Overall, dauer larvae are transcriptionally repressed compared with actively growing, non-dauer larva stages [3
]. However, the expression of some genes is comparatively enhanced in, or specific to, dauer larvae [3
], showing that this transcriptional repression does not apply to all genes. If environmental conditions 'improve' then dauer larvae resume their development via
the L4 stage. Thus, the decision whether to develop into dauer larvae or into 'normal', non-dauer larvae is environmentally determined. The dauer or non-dauer developmental programme will, at least in part, be executed by transcriptional control.
The features of the environment that are used by larvae making this developmental decision are the concentration of food, the concentration of dauer pheromone and temperature. Dauer pheromone is a cue produced by all worms that acts as a measure of con-specific population density [7
] and appears to consist of at least three related molecules [8
]. Conditions that favour the development of dauer larvae are a low concentration of food and a high concentration of dauer pheromone (i.e
. a high conspecific population density). Conversely, conditions that favour the development of non-dauer larva development are a high concentration of food and a low concentration of dauer pheromone. Higher temperatures favour the development of dauer larvae [1
There has been extensive investigation into the genetic and molecular genetic control of the development of dauer larvae, which is known to be controlled by a TGF-β-like pathway, an insulin-like pathway and a guanyl cyclase pathway [1
]. There have been a number of studies that have compared gene expression in dauer larvae with other life-cycle stages [5
], compared L2, L3 and dauer larvae of wild type and mutant lines [6
] or determined how gene expression changes during entry into the dauer larva stage [11
]. These studies have found large differences in the transcriptional profiles of these stages, fully consistent with the different morphology and physiology of dauer larvae. Genes involved in the insulin-like pathway, particularly the FOXO-family transcription factor daf-16
, have been shown to be key in the generation of these transcriptional differences [12
]. However, these studies have not investigated variation in gene expression between isolates nor the very early stages of the dauer/non-dauer larva decision. At these early stages it can be envisaged that there may be small differences that initiate subsequent larger transcriptional changes. In this sense, previous studies have investigated changes in gene expression that are associated with dauer development rather than the genes that are involved in making the decision between dauer and non-dauer larval development.
The natural history of C. elegans
is still poorly understood. However, individual C. elegans
are most often isolated from the wild as dauer larvae, rather than as reproducing adults [15
]. This observation suggests two things: firstly, that the dauer larva morph is of central importance in the natural history of C. elegans
and, secondly, that dauer larvae and the developmental decision whether or not develop into dauer larvae is likely to be under strong natural selection. Previously we have compared the plasticity of dauer larva development of different lines of C. elegans
]. Plasticity is a measure of the sensitivity of lines to different environmental conditions, with this sensitivity measured as the difference in the proportion of larvae that develop into dauer larvae between two or more different environments. We have found that lines of C. elegans
differ in their plasticity of dauer larva development [16
]. For example, over a range of concentrations of dauer pheromone, in some lines, only a few individuals will develop as dauer larvae (i.e
. low plasticity lines); in other lines, the proportion of individuals that develop as dauer larvae will increase rapidly with the concentration of dauer pheromone (i.e
. high plasticity lines).
Genetic analysis of variation in the plasticity of dauer larvae formation has identified a number of quantitative trait loci (QTL) that control it [17
]. Given that transcriptional differences are also likely to be involved in dauer larva development we wished to determine whether such transcriptional differences originated from these QTL regions. More particularly, we hypothesised that inter-line differences in the phenotypic plasticity of dauer larvae development of C. elegans
is due to inter-line transcriptional differences. To investigate this we have investigated the transcriptional profiles of C. elegans
lines with different phenotypic plasticities of dauer larva development. We further hypothesised that the different dauer larva development plasticities were most likely to be due to interline differences in the 'decision' and early initiation of development into non-dauer or dauer larvae. For this reason we compared the transcriptional profile of early stage larvae exposed to dauer larva or 'normal', non-dauer larva-inducing conditions.