Conifers are the largest, most widely distributed group of gymnosperms in the world. Conifers are predominantly diploid. Polyploidy, which is widespread in angiosperms, is rare in conifers. There are only a few naturally occurring polyploids in conifers. Of the eight families of conifers, natural polyploids have been reported only in four genera of Cupressaceae, namely, Sequoia, Fitzroya, Juniperus, and Xanthocyparis. Sporadic and artificially induced polyploids in confers show abnormal growth and do not develop into mature trees. It is conceivable that polyploidy may not have been a viable mechanism for the evolution of predominantly diploid conifers. The large genome size in confers probably resulted from gene duplications, and proliferation of repetitive DNA and transposable elements. The question is how conifers have achieved a genetically stable diploid genetic state after their genome expansion. in most of their families? Creation of polyploidy in organisms is largely determined by reproductive errors involving production of unreduced gametes. In this review I examine the question of rarity of polyploidy in conifers and propose that the maintenance of the diploid state was accomplished by a constellation of diploid-specific genome stability genes and/or non-coding elements in this group. These genome stability genes presumably regulate: (1) predominantly stable haploid gamete formation (rare unreduced gametes result in sporadic abnormal polyploid trees); and (2) an efficient DNA double-strand break repair mechanism for elimination of non-adaptive mutations in conifers.
