Unusual pattern of bacterial ice nucleation gene evolution

Bacterial ice nucleation activity (INA+ phenotype) can be traced to theproduct of a single gene, ina. A remarkably sparse distribution of thisphenotype within three bacterial genera indicates that the ina gene mayhave followed an unusual evolutionary path. Southern blot analyses, coupledwith assays for ice-nucleating ability, revealed that within four bacterialspecies an ina gene is present in some strains but absent from others.Results of hybridization experiments using DNA fragments that flank the inagene suggested that the genotypic dimorphism of ina may be anomalous. Aphylogenetic analysis of 16S ribosomal RNA gene sequences from a total of14 ina+ and ina- bacterial strains indicated that the ina+ bacteria are notmonophyletic but instead phylogenetically interspersed among ina- bacteria.The relationships of ina+ bacteria inferred from ina sequence did notcoincide with those inferred from the 16S data. These results suggest thepossibility of horizontal transfer in the evolution of bacterial ina genes.