Contributions to Zoology, 86 (4) – 2017Jacques J. M. van Alphen; Jan W. Arntzen: The case of the midwife toad revisited

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The crossing experiments reported in Kammerer (1911, 1924) are between water-breeding and land-breeding midwife toads. Vargas et al. (2016) have based their explanation on a single gene interpretation and conclude on a “parent of origin effect”. They say this “is found in epigenetic mechanisms that selectively inactivate a gene in only one of the sexes”. However, “land-breeding” and “water-breeding” are, in Kammerer’s description, adaptations in a whole suite of characters, including: (1) the behaviour to choose a mating site, (2) absence or presence of male parental care, (3) absence or presence of nuptial pads, (4) resistance of eggs to Oomyceta, (5) egg size, (6) egg morphology, (7) female fecundity, (8) the developmental stage at which larvae hatch from the eggs, and (9) age at first reproduction. It is hard to believe that all these traits are the expression of a single gene. Similarly, there is no reason to assume that the multiple genes involved in these characters would be tightly linked as to form a “supergene”. Kammerer (1911) used egg size and fecundity to recognize water- from land-breeding females, and presence or absence of male parental care to recognize water- from land-breeding males. These are very different traits, unlikely to be coded for by the same gene. By using egg-size and fecundity to classify the females as land- or water-breeding, Kammerer tacitly assumed that these traits were completely linked to the operational breeding mode, like the presence or absence of male parental care. However, if we assume the most simple genetic architecture with just two genes segregating independently, with two alleles each, one gene for male parental care and one gene for egg size, then, any experimenter in charge would classify no more than 25% of the females correctly as land-breeding or water breeding.

Heritability and QTL studies in animal groups as different as birds (Bentz et al., 2016; Yi et al., 2015; Santure et al., 2013), fish (Huppop & Wilkens, 1991), bristleworms (Miles et al., 2007) and insects (Bauerfeind & Fischer, 2007) have shown that egg size and female fecundity are complex quantitative traits influenced by many genes positioned on different chromosomes, as is the case for development and other life history traits in the Amphibia (Voss et al., 2012; Hangartner et al., 2012). Any more complicated genetic architecture would render the classifications even less accurate as the relation between egg size and breeding mode weakens further when more genes are involved. Therefore, the numbers provided by Kammerer (1911) do not genuinely represent numbers of land- and water-breeding animals and, thus, it cannot be concluded that one quarter of the F2 -offspring belonged to one and three quarters to the other breeding model. As egg-size and fecundity are determined by a number of quantitative trait loci on different chromosomes, egg size and number should show continuous variation. This would have created an additional problem for Kammerer (1911) in classifying animals of the F2 generation as water- or land-breeding. However, Kammerer’s (1911) data on egg-size and number are strongly bimodal and such is hard to explain given the genetic architecture of the traits. Hence, the data provided by Kammerer do not provide support for a role of “parent of origin” effect and epigenetics in explaining Kammerer’s enigmatic claims.