Telegony, the Sire Effect and non-Mendelian Inheritance Mediated by Spermatozoa: A Historical Overview and Modern Mechanistic Speculations

Uterosomes: The lost ring of telegony?

Telegony refers to the appearance of some characteristics of the female's previously mated male in her subsequent offspring by another male. According to evidence, telegony may occur either through the infiltration of sperm into the somatic tissues of the female genital tract or the presence of fetal genes in the mother's blood. It is highlighted that sperm penetrates into the mucosa of the uterine and possibly alters the genetic structure, affecting the embryo and enduring from one pregnancy to the next, which may be one of the potential mechanisms of telegony. Uterine fluid, uterine gland-derived histotroph, supplies key nutrients for successful embryo implantation and it is important during the first trimester, especially, because of its susceptibility to maternal states. The presence of EVs in uterine fluid (uterosomes) was reported in mice, sheep, and humans, including a wide range of biomolecules, such as proteins, and non-coding RNAs. In this review article, we presented a new idea to explain telegony. Based on our idea, after the previous male sperm entry into the female reproductive system, those sperm which do not participate in fertilization penetrate into the somatic cells of the uterus and store their genetic/epigenetic information there. The sperm of the next partner reaches a location in the female reproductive canal where it exchanges information with the uterosomes and obtains the proteins and non-coding RNAs required for fertilization, development, and implantation.

Mapping the past, present and future research landscape of paternal effects

BACKGROUND

Although in all sexually reproducing organisms an individual has a mother and a father, non-genetic inheritance has been predominantly studied in mothers. Paternal effects have been far less frequently studied, until recently. In the last 5 years, research on environmentally induced paternal effects has grown rapidly in the number of publications and diversity of topics. Here, we provide an overview of this field using synthesis of evidence (systematic map) and influence (bibliometric analyses).

RESULTS

We find that motivations for studies into paternal effects are diverse. For example, from the ecological and evolutionary perspective, paternal effects are of interest as facilitators of response to environmental change and mediators of extended heredity. Medical researchers track how paternal pre-fertilization exposures to factors, such as diet or trauma, influence offspring health. Toxicologists look at the effects of toxins. We compare how these three research guilds design experiments in relation to objects of their studies: fathers, mothers and offspring. We highlight examples of research gaps, which, in turn, lead to future avenues of research.

CONCLUSIONS

The literature on paternal effects is large and disparate. Our study helps in fostering connections between areas of knowledge that develop in parallel, but which could benefit from the lateral transfer of concepts and methods.

Offspring phenotype is shaped by the nonsperm fraction of semen

In a large majority of animal species, the only contribution of males to the next generation has been assumed to be their genes (sperm). However, along with sperm, seminal plasma contains a wide array of extracellular factors that have many important functions in reproduction. Yet, the potential intergenerational effects of these factors are virtually unknown. We investigated these effects in European whitefish (Coregonus lavaretus) by experimentally manipulating the presence and identity of seminal plasma and by fertilizing the eggs of multiple females with the manipulated and unmanipulated semen of several males in a full-factorial breeding design. The presence of both own seminal plasma and foreign seminal plasma inhibited sperm motility, and the removal of own seminal plasma decreased embryo survival. Embryos hatched significantly earlier after both semen manipulations than in control fertilizations; foreign seminal plasma also increased offspring aerobic swimming performance. Given that our experimental design allowed us to control potentially confounding sperm-mediated (sire) effects and maternal effects, our results indicate that seminal plasma may have direct intergenerational consequences for offspring phenotype and performance. This novel source of offspring phenotypic variance may provide new insights into the evolution of polyandry and mechanisms that maintain heritable variation in fitness and associated female mating preferences.

The Criticisms of Pangenesis: The Years of Controversy

When first published in 1868, Darwin's Pangenesis was almost uniformly rejected by his contemporaries. Until recently it has still been regarded as Darwin's biggest mistake or a brilliant blunder. There are three main reasons for this. First, Galton transfused the blood of one variety of rabbit into another, and then bred together the latter. The results of breeding showed no variations of characters in the offspring. Thus he concluded that Darwin's Pangenesis was incorrect. Second, there was no direct evidence for the existence of Darwin's imaginary gemmules. Third, Darwin's Pangenesis explained the Lamarckian inheritance of acquired characters, graft hybridization, xenia and telegony, which were largely thought to be doubtful phenomena. Now the discoveries of circulating cell-free DNA, mobile RNAs, prions and extracellular vesicles provide striking evidence for the chemical existence of Darwin's supposed gemmules. There is also convincing evidence for heritable changes induced by blood transfusion in which Galton failed to find such effects in his experiment. Moreover, there is increasing evidence for the inheritance of acquired characters, graft hybridization, xenia and other phenomena that Pangenesis was designed to explain. In light of the mounting evidence, it is not proper to continue to consider Pangenesis as Darwin's biggest mistake or a brilliant blunder.

Darwin's Pangenesis and Certain Anomalous Phenomena

Darwin clearly described certain anomalous phenomena, including what he referred to as "the direct action of the male element on the female form" and what we now call xenia and telegony, bud variation (mutation), reversion or atavism, and the inheritance and non-inheritance of mutilation. Some phenomena, particularly xenia, telegony and the inheritance of mutilation, were considered as doubtful phenomena by such authorities as Weismann and Morgan. Over the past 150 year, however, there has been increasing evidence for xenia, which is of great interest and importance in physiological research and plant production. The discoveries of cell-free fetal DNA, sperm RNAs, penetration of sperm into the somatic tissues of the female reproductive tract and the incorporation of exogenous DNA into somatic cells indicate that molecular mechanisms exist for telegony, one of the most controversial issues. Darwin's Pangenesis is the only theory that explains all the different types of phenomena.

Darwin and Mendel today: a comment on “Limits of imagination: the 150th Anniversary of Mendel’s Laws, and why Mendel failed to see the importance of his discovery for Darwin’s theory of evolution”

We comment on a recent paper by Rama Singh, who concludes that Mendel deserved to be called the father of genetics, and Darwin would not have understood the significance of Mendel’s paper had he read it. We argue that Darwin should have been regarded as the father of genetics not only because he was the first to formulate a unifying theory of heredity, variation, and development — Pangenesis, but also because he clearly described almost all genetical phenomena of fundamental importance, including what he called “prepotency” and what we now call “dominance” or “Mendelian inheritance”. The word “gene” evolved from Darwin’s imagined “gemmules”, instead of Mendel’s so-called “factors”.

Revisiting telegony: offspring inherit an acquired characteristic of their mother's previous mate

Newly discovered non-genetic mechanisms break the link between genes and inheritance, thereby also raising the possibility that previous mating partners could influence traits in offspring sired by subsequent males that mate with the same female ('telegony'). In the fly Telostylinus angusticollis, males transmit their environmentally acquired condition via paternal effects on offspring body size. We manipulated male condition, and mated females to two males in high or low condition in a fully crossed design. Although the second male sired a large majority of offspring, offspring body size was influenced by the condition of the first male. This effect was not observed when females were exposed to the first male without mating, implicating semen-mediated effects rather than female differential allocation based on pre-mating assessment of male quality. Our results reveal a novel type of transgenerational effect with potential implications for the evolution of reproductive strategies.

Fetal genes in mother's blood: a novel mechanism for telegony?

Telegony is a discredited genetic phenomenon that a previous male may influence the characteristics of offspring subsequently borne by the same female to another male. Although its reality was acknowledged by such authorities as Charles Darwin and Herbert Spencer, it has been met with skepticism because of a lack of understanding of the theoretical basis for telegony. With the discovery of fetal genes in mother's blood, the penetration of somatic cells by sperm, and the ability of RNA to program genome rearrangement, mechanisms might exist for telegony.