Breaching the Weismann barrier – a role of vitellogenin yolk proteins in nematode non-Mendelian inheritance

August Weismann from Freiburg, Germany played an essential role in the general acceptance of Charles Darwin´s theory of evolution. He argued against the inheritance of acquired characters and formulated the idea of the continuity of the germ plasm within and between generations. This separation of germ line and soma – generally referred to as the Weismann barrier – is still of conceptional importance in contemporary biology. However, many new findings in modern biology contradict this perspective. In response to environmental perturbations, various forms of non-Mendelian inheritance, either intergenerational or transgenerational, are observed in many plants and animals. In particular, work in the nematode models Caenorhabditis elegans and Pristionchus pacificus with their short generation time, simple husbandry and self-fertilizing mode of reproduction resulting in isogenic cultures, provide examples of transgenerational memory, often for multiple generations. Known associated molecular processes involve a plethora of mechanisms including among others, small RNA signalling and histone modifications. In this review, we will highlight the potential role of vitellogenin (yolk) proteins in memory transmission. While only rarely considered, yolk proteins represent a major exception to the Weismann barrier as they are synthesized in the soma and are specifically transported into the germ line. We will discuss recent findings that suggest intergenerational and transgenerational inheritance to rely on vitellogenin and will speculate on molecular mechanisms that might allow breaching the Weismann barrier. Finally, we will discuss the potential role of transgenerational inheritance for plasticity-associated evolution, a phenomenon that has been introduced in the last decade as a major facilitator of evolutionary diversification and novelty.