Leszek Berger
Throughout his research career, Berger took extensive notes on all of the innumerable crosses that he made among water frogs.
This distinctive reproductive system, termed hybridogenesis, (Schultz 1969) was otherwise known only in fresh-water fishes of the genus Poeciliopsis, which are primarily Mexican and Central American.
Prof. Berger's observation (associated with the numerous laboratory crosses that he made) that P. esculentus females frequently produce ova with distinct size classes led to examination of red blood cell sizes of adult frogs used for crosses and the discovery of abundant triploid hybrid water frogs, first in northern Poland and later in northern Germany as well.
Morphological data, later confirmed by allozyme electrophoresis, suggested that triploids were present in two distinct forms, and indeed those more resembling marsh frogs contained two complete genomes of that species and one of P. lessonae; whereas those more resembling pool frogs contained two complete genomes of that species and one of P. ridibundus.
Later crosses revealed the array of gametes produced by these two morphological types (Günther, Uzzell, and Berger 1979).
Utilizing the abundant data that he regularly collected over several years on three populations from near Turew, Wielkopolska (numbers of adults per kind, numbers of eggs and egg masses produced, numbers fertilized, numbers and kinds of tadpoles produced) Berger offered convincing evidence that the single all-hybrid population among the three persisted, not because of the usual matings that permit hybrid water frog lineages to persist, but only because of rare successes of fertilization involving ova with several unusual and rare genomic compositions.
In 1973 Prof. Berger received the first-degree award from the Polish Academy of Science for his discovery of a new type of heredity and reproduction in one of the most common European amphibian species.
Significant collaborators (5 or more publications) include Stefania Bucci (IT), Rainer Günther (GE), Elżbieta Czarniewska (PL), Hansjürg Hotz (CH), Giorgio Mancino (IT), Matilda Raghianti (IT), Mariusz Rybacki (PL), and Thomas Uzzell (USA).
Today Professor Berger's research is being continued in the Department of Evolutionary Biology and Conservation of Vertebrates at the University of Wrocław (Maria Ogielska team).
Over the course of 39 years (1963–2001) Prof. Berger bred and crossed all 16 taxa of western Palearctic water frogs.
He obtained over 800,000 offspring from approximately 1,500 crosses, making him perhaps the most prolific scientific breeder of amphibians in the world.
An all-hybrid water frog population persisting in agrocenoses of central Poland (Amphibia, Salientia, Ranidae).
