Lysis time, optimality, and the genetics of evolution in a T7 phage model system

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Title: Lysis time, optimality, and the genetics of evolution in a T7 phage model system
Author: Heineman, Richard Hugh, 1978-
Abstract: The ability of traits to adapt in response to change is one of the most fundamental aspects of evolution . Optimality models used to predict adaptation frequently make simplifying assumptions about the ability of traits to evolve freely within simple tradeoffs . However , we frequently have little understanding of genomic mechanisms underlying phenotypic evolution . Genetic constraints clearly limit phenotypic change , but the extent to which they do so is unclear . I will explore molecular and phenotypic responses to genomic and environmental perturbations through experimental evolution in T7 bacteriophage . First , I studied evolutionary robustness of the lysis time phenotype when lysin gene lysozyme was deleted . This deletion profoundly delayed lysis and thus decreased fitness . Evolved phages recovered much of the lost fitness and mostly restored lysis timing . The recovery was mediated by changes in a tail fiber gene (gene 16 ) with muralytic activity that is generally used in genome entry . Next , I extended the work on lysozyme to observe the effect of increasing constraint on evolutionary recovery . The effects of various combinations of deletions of lysozyme , 17 .5 (which plays a role in lysis ) and 16 suggested that another gene played a role similar to 17 .5 in lysis . The phage defective in both lysozyme and 16 did not lyse hosts thoroughly even after long periods of infection , suggesting that these were the only effective lysin genes . Adaptation of this phage on cells expressing the essential gp16 constrained the primary adaptive pathway of recovery from lysozyme deletion . A mutually exclusive alternative pathway involving a variety of different genes evolved . The line recovered the ability to lyse normal hosts , by a mechanism involving multiple mutations . Finally , I tested the ability of T7 to adapt to an optimum lysis time . Based on empirical results from other phages , mature phage virions accumulate linearly inside the cell over time . This assumption underlies a model suggesting that availability of hosts determines optimal lysis time . While adaptation to different host densities caused the expected qualitative evolutionary changes , adaptation to conditions expected to select for slow lysis did not lead to the quantitative optimum . This is probably due to nonlinear virion accumulation .
URI: http : / /hdl .handle .net /2152 /3439
Date: 2008-08-28


Lysis time, optimality, and the genetics of evolution in a T7 phage model system. Doctoral dissertation, The University of Texas at Austin. Available electronically from http : / /hdl .handle .net /2152 /3439 .

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