These three mechanisms are instances of the general Hill-Robertson effect that describes the reduction in the efficiency of selection in the presence of segregating mutations under selection when recombination is either absent or reduced. In the third phase of sex chromosome differentiation, three different processes-Muller's ratchet, background selection, and genetic hitchhiking-may contribute to degeneration of the Y (or W) chromosome once recombination is reduced in all or part of the nascent sex-specific chromosome. Close linkage between sexually antagonistic variation and the sex-determining gene has been proposed to start Y chromosome morphological differentiation from the X chromosome. We conclude by looking at ways to test the hypothesis that palindromes enhance the rate of adaptive evolution of Y-linked genes and whether this effect can be extended to palindromes on other chromosomes. This hypothesis helps reconcile two enigmatic features of the “palindromes as protectors” view: (1) genes that are not located in palindromes have been retained under purifying selection for tens of millions of years, and (2) under models that only consider deleterious mutations, gene conversion benefits duplicate gene maintenance but not initial fixation. Our paper emphasizes the latter, that palindromes may exist to accelerate adaptation by increasing the potential targets and fixation rates of incoming beneficial mutations. However, the adaptive significance of recombination resides in its ability to decouple the evolutionary fates of linked mutations, leading to both a decrease in degeneration rate and an increase in adaptation rate. Since palindromes enable intrachromosomal gene conversion that can help eliminate deleterious mutations, they are often highlighted as mechanisms to protect against Y degeneration. Y chromosome palindromes consist of inverted duplicates that allow for local recombination in an otherwise nonrecombining chromosome. Send us feedback about these examples.We look at sex-limited chromosome (Y or W) evolution with particular emphasis on the importance of palindromes. These examples are programmatically compiled from various online sources to illustrate current usage of the word 'palindrome.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. 2021 The next date to feature a five-digit palindrome with only one number will occur next century, on Jan. 2011 Thursday marks an eight-digit palindrome, which occurs only 12 times this century. 2023 Interview with, and essay by, palindrome king Barry Duncan. Richard Lederer, San Diego Union-Tribune, 10 June 2023 The Clue: This word is a palindrome. 2022 Manufactured by Honda, the Civic automobile is a palindrome composed entirely of Roman numerals. Wyna Liu, New York Times, 26 June 2023 Twosday is a global celebration, a set-in-stone palindrome no matter the date format. 2021 That’s where the puzzle element could come in: Some categories might be defined by their use of wordplay - palindromes, homophones, adding or dropping letters and words - rather than the literal meanings of the words on the cards. Recent Examples on the Web The most significant palindrome date will be the one from Inauguration day, which also marks the start of 10 consecutive days of palindromes.
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