HIGH-FREQUENCY INTRAGENIC RECOMBINATION DURING MACRONUCLEAR DEVELOPMENT IN TETRAHYMENA-THERMOPHILA RESTORES THE WILD-TYPE SERH1 GENE

Macronuclear development in ciliates is characterized by extensive rea rrangement of genetic material, including sequence elimination, chromo some fragmentation and telomere addition. Intragenic recombination is a relatively rare, but evolutionarily important phenomenon occurring i n mitosis and meiosis in a wide variety of organisms. Here, we show th at high frequency intragenic recombination, on the order of 30%, occur s in the developing amitotic macronucleus of the ciliate Tetrahymena t hermophila. Such recombination, occurring between two nonsense transit ion mutations separated by 726 nucleotides, reproducibly restores wild -type expression of the SerH1 surface protein gene, thus mimicking com plementation in trans heterozygotes. Recombination must be considered a potentially important aspect of macronuclear development, producing gene combinations not present in the germinal micronucleus.