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From page 165... ... Although direct gene transfer between species has occurred at some level in all major groups, it appears to be far more frequent in prokaryotes than in multicellular eukaryotes. An alternative to incorporating novel genes into a recipient genome is acquiring a stable, possibly heritable, symbiotic association and thus enjoying benefits of complementary metabolic capabilities. Read the entire page →
From page 166... ... Thus, intimate association between two lineages can readily arise through natural selection acting within each species to fix alleles that promote close association with the other species. Although differences in metabolic capacities among species have long been evident, genomics is Read the entire page →
From page 167... ... . As a group, animals are unusual in lacking the ability to produce numerous universally required metabolic compounds such as vitamins and amino acids and also in lacking capabilities for producing bioactive secondary compounds that can act as toxins and defenses. Read the entire page →
From page 168... ... . in Bacteria, such incorporation of foreign genes is the major route to the origination of novel capacities (ochman et al., 2000) Read the entire page →
From page 169... ... Because of the different capabilities of different species, mutually beneficial associations can arise de novo from organisms that are not coevolved, and these associations can then become stabilized through natural selection acting within each species. Mutual advantage often can be enhanced by natural selection when the two lineages are associated across generations, although heritability of the symbiosis is not a require ment for mutual benefit (sachs et al., 2004) Read the entire page →
From page 170... ... . SYMBIOTIC MICROBIAL CONSORTIA one consequence of the fact that gene transfer is not without limits even in prokaryotes is the frequent evolution of microbial consortia or microbial syntrophy, that is, close, often obligate, associations of two (or a few) Read the entire page →
From page 171... ... and complicated histories of gene transfers among several genomes (Keeling, 2004) Read the entire page →
From page 172... ... Duplication of existing genes and regulatory changes are far more important. Among potential barriers to gene acquisition in animals are the need for regulating expression in the context of the more complex development and also the separation of germ line. Read the entire page →
From page 173... ... one basis of the abundance of symbiotic interactions in this group is the poor diet of most species: plant phloem sap and xylem sap are both particularly unbalanced nutritionally, lacking essential amino acids, and, in the case of xylem sap, vitamins and carbohydrates. Thus, a phloem sap- or xylem sap-feeding animal, while enjoying the advantage of a constant food supply, must collaborate with a microbial symbiont able to synthesize missing nutrients from precursors that are available. Read the entire page →
From page 174... ... sharpshooters are distinguished from related insects in that they have adopted a xylem sap diet, imposing distinct nutritional Read the entire page →
From page 175... ... colonized at different stages in sharpshooter evolution; they provide nutrients needed to supplement the xylem sap diet (Moran et al., 2005b; Takiya et al., 2006; Wu et al., 2006) Read the entire page →
From page 176... ... . As for Buchnera, Sulcia possesses a very small genome but retains pathways for synthesis of most essential amino acids, nutrients that are in short supply in both phloem and xylem sap, in both of which amino acid profiles are dominated by nonessential amino acids. Read the entire page →
From page 177... ... . GENOMIC DECAY IN OBLIGATE SYMBIONTS AND HOST DEPENDENCE obligate nutritional symbionts of insects provide prime examples of genome degradation in obligately host-associated bacterial lineages. Read the entire page →
From page 178... ... The extent of gene transfer from symbionts to animal hosts will become appar Read the entire page →
From page 179... ... Although the bacteriome-associated nutritional symbionts provide the most extreme cases of genome stasis known in Bacteria and do not acquire novel genes (Tamas et al., 2002; Degnan et al., 2005) , some heritable bacteria continue to undergo recombination, to harbor phage, and to incorporate foreign genes into their chromosomes. Read the entire page →
From page 180... ... examples from insects show that symbioses can result in specialized organs with unique development, innovations in metabolic capabilities that allow new lifestyles, defenses against natural enemies and other environmental challenges, constraints on evolutionary range, and ongoing acquisition of novel genes and capabilities. Read the entire page →
From page 181... ... Symbiosis as an Adaptive Process and Source of Phenotypic Complexity / ACKNOWLEDGMENTS i thank Francisco Ayala and John Avise for organizing this conference, howard ochman for critical comments on a draft of the manuscript, Daniela Takiya and roman rakitov for use of their photograph, Becky nankivell for preparation of the manuscript and Fig. 9.1, and two reviewers for helpful comments. Read the entire page →

From page 165...

... Although direct gene transfer between species has occurred at some level in all major groups, it appears to be far more frequent in prokaryotes than in multicellular eukaryotes. An alternative to incorporating novel genes into a recipient genome is acquiring a stable, possibly heritable, symbiotic association and thus enjoying benefits of complementary metabolic capabilities.