UNILATERAL INCONGRUITY IN CROSSES INVOLVING LYCOPERSICON PENNELLII AND LYCOPERSICON-ESCULENTUM IS DISTINCT FROM SELF-INCOMPATIBILITY IN EXPRESSION, TIMING AND LOCATION

Both interspecific and intraspecific mechanisms restrict the exchange of genes between plants. Much research has focused on self incompatibi lity (SI), an intraspecific barrier, but research on interspecific bar riers lags behind. We are using crosses between Lycopersicon esculentu m and L. pennellii as a model with which to study interspecific crossi ng barriers. The cross L. esculentumL. pennellii is successful, but th e reciprocal cross fails. Since the cross can be successfully made in one direction but not the other, gross genomic imbalance or chromosoma l abnormality are precluded as causes. We showed that the lack of seed set observed in the cross L. pennelliixL. esculentum is due to the in ability of pollen tubes to grow more than 2-3 mm into the style, where as SI crosses show continued slow pollen tube growth but, also, fail t o set seed. These results indicate that the unilateral response is a b arrier distinct from SI, differing from SI in the timing and location of expression in the style. We therefore suggest that this unilateral response in the L. pennelliixL. esculentum cross is more accurately re ferred to as ''unilateral incongruity'' (UI) rather than interspecific incompatibility. Periclinal chimeras were used to determine the tissu es involved in UI. The results of crosses with the available chimeras indicate that the female parent must be L. pennellii at either LI (lay er 1) or both LI and LII (layer 2) and the male parent must be L. escu lentum at either LII or both LI and LII to observe UI similar to that seen in the L. pennelliixL. esculentum cross. Pollinations with a mixt ure of pollen from L. pennellii and from transgenic L. esculentum plan ts harboring a pollen-specific GUS reporter gene marker were used to a scertain whether the growth of the pollen tubes of either species was modified as a possible means of overcoming UI. We found no evidence of communication between the two types of pollen tubes to either enhance or restrict all pollen tube growth.