To better understand the role of gene-environment interactions in development and evolution, we study quantitative natural genetic variation and plasticity of the Caenorhabditis germline and reproductive system in different environments. Although the basic germline organization and processes are conserved among Caenorhabditis species, reproductive output and schedules vary both within and between species. We are interested in understanding how differences in offspring number and quality are coupled to the underlying properties and processes of the germline (e.g. sperm number, germ cell number, proliferation, apoptosis) and to what extent they are plastic, i.e. vary across different (ecologically relevant) environments. In an initial analysis, we have quantified germline and reproductive phenotypes in 15 isolates of the three hermaphroditic species (C. elegans, C.briggsae and C. sp. 11). Overall, sperm number, germ cell number and offspring number are positively correlated; however, sperm number does not always closely match offspring number, indicating that isolates and species may differ in sperm fertility or efficiency of sperm use. The most striking observation is that many C. sp. 11 isolates show a highly reduced offspring, sperm, germ cell number and mitotic zone relative to C. elegans. To carry out an integrative analysis of germline and reproductive plasticity, we have characterized how e.g. germ cell proliferation, entry into meiosis and apoptosis are modified in N2 animals exposed to diverse conditions (such as liquid, starvation, different bacterial food sources, ethanol, acetic acid, temperature shifts, osmotic or hypoxia). Our results confirm that diverse germline rocesses are highly environmentally sensitive. We also show that stressful conditions may reduce offspring number through reduction of either sperm fertility or number, as well as defects in germline progression. The plastic responses in reproductive features of C. elegans N2 may differ greatly from the ones observed in other wild isolates of C. elegans,C. briggsae or C. sp. 11, revealing considerable genotype-by-environment interactions. We will discuss these and other results in the context of how such differential plasticity of the reproductive system contributes to germline integrity and reproductive success in variable environments.