Selection for increased numbers of extra heterochromatic chromosomes in the tomato.

Crossing two parental plants carrying two types of extra chromosomes, 2n+2(5L*7S) by 2n+3(2S*2S) permitted effective selection for individuals with up to eight 2S*2S chromosomes in later generations. The crossing of 2n + 3(2S*2S) by 2n+1( 8S*8L) and by the wild tomato relative, diploid L. pimpinellifolium , produced plants with up to four extra chromosomes. The tolerance of 2S*2S chromosomes through both gametophytes is at least four extras. The presence of two to eight extra 2S*2S chromosomes decreased pollen fertility between 82 to 41%. Progeny tests reveal that 2S*2S chromosomes tend to be lost in transmission. Progenies of self-pollinated individuals with 25, 26, 27, 28, 29 and 30 chromosomes averaged 24.3, 25.1, 25.5, 27.1, 28.2 and 27.9 chromosomes respectively. The offspring of crosses between diploid and extrachromosomal plants showed similar tendencies. Evidently tomato 2S*2S chromosomes lack the mechanisms for accumulation characteristic of some plant species with naturally occurring accessory chromosomes.-Extra 2S*2S chromosomes are very stable mitotically and are not eliminated from the somatic tissue of unmodified plants. Severe pruning of certain plants induced loss or gain of these chromosomes.-The importance of accumulating two to 18 nucleolar organizer regions in a cell by addition of 2S*2S chromosomes is discussed.