6 Heterogenic Incompatibility in Fungi
In biology, incompatibility is usually defined as restriction of mating competence controlled by genes other than those determining sexual differentiation. It has been long recognised that incompatibility concerns not only the sexual phase but also the ve
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Heterogenic Incompatibility in Fungi
K. ESSER1
CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 II. Barrage Formation . . . . . . . . . . . . . . . . . . . . . . . . . . 104 III. Heterogenic Incompatibility in the Ascomycete Podospora anserina . . . . . . 109 A. The Phenomenon . . . . . . . . . . . . . . . . . . . . . . . . . 109 B. Genetic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 C. Physiological Expression . . . . . . . . . . . . . . . . . . 111 D. Function of the het-Genes . . . . . . . . . . . . . . . . 112 IV. Further Examples of Heterogenic Incompatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 A. Oomycota . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 B. Glomeromycota . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 C. Dikaryomycota . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 V. Correlations with Heterogenic Incompatibility in Plants and Animals, with DNA Restriction in Bacteria and with Histoincompatibility . . . 122 VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
I. Introduction In biology, incompatibility is usually defined as restriction of mating competence controlled by genes other than those determining sexual differentiation. It has been long recognised that incompatibility concerns not only the sexual phase but also the vegetative phase. The latter becomes apparent especially in fungi and was first termed heterokaryon incompatibility. In both sexual and vegetative incompatibility, the action of the genetic traits involved precludes the exchange of genetic material. Thus, inhibition of recombination results by a lack of karyogamy (sexual incompatibility) as well as by an inability of the nuclei to coexist in a 1
Lehrstuhl fu¨r Allgemeine Botanik, Ruhr-Universita¨t Bochum, 44780 Bochum, Germany; e-mail: [email protected]
common cytoplasm and to undergo somatic recombination (vegetative incompatibility). Since recombination is of paramount importance in evolution, the biological significance of incompatibility as a factor controlling recombination is immediately apparent. Nature has evolved two principal systems to control incompatibility. According to their mode of genetic determination, these have been called homogenic and heterogenic incompatibility (Esser 1962). The genetic basis of homogenic incompatibility consists in a sexual incompatibility of nuclei carrying identical incompatibility factors. Heterogenic incompatibility consists in a genetic difference of at least one single gene which inhibits the coexistence of the nuclei concerned in a common cytoplasm. From these definitions, it follows that homogenic incompatibility enhances outbreeding and favours recombination and evolution of the species. Heterogenic incompatibility, however, restricts outbreeding and thereby favours the evolution of isolated groups within a single spe
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