Scientists have recently sequenced the complete genomes of four of these columnar cacti, and found, to their surprise, that their family relationships are not so straightforward as their shapes suggest. According to Noah Whiteman, a UC Berkeley associate professor of integrative biology who is a coauthor of a paper appearing this week in the journal Proceedings of the National Academy of Sciences, the cactus family tree and the giant cacti in particular – the giant saguaro, organ pipe, senita and cardón, also called the Mexican giant cactus – have been very difficult to trace. In the study, led by Whiteman’s colleagues at the University of Arizona and the Universidad Nacional Autónoma de México, the scientists created individual family trees of each gene shared across all species. They found that their histories were scrambled as a result of long generation times – saguaro cacti can live 150 years or more – making it difficult to understand the relationships among the species even with complete genomic information. They did determine, however, that some similarities, like the succulent flesh that makes some cacti a good emergency source of water, resulted from ancient genes that were retained by some cacti but lost by others. What looked like parallel evolution, with some species gaining new genes and new functions, was actually just the random loss of genes in all the other species. The findings could have implications for the fate of these cacti, which are losing habitat because of human development in arid areas of the Americas.
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