Accelerated Gene Evolution and Subfunctionalization in the Pseudotetraploid Frog Xenopus Laevis Page: 5 of 50
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remaining loci, at least in the early stages of rediploidization. As
haploinsufficiency is relatively rare , reduced copy number is not by itself an
overwhelming impediment to large scale loss, as is evident from analysis of
surviving duplicates in the Arabidopsis, rice, teleost, and yeast genomes [9,11-
Early thoughts on the selective forces leading to duplicate gene retention
centered on divergence in protein function. This suggests that one or both copies
could acquire novel  and/or complementary  biochemical functions that
would render both copies indispensable. It was further recognized that novel or
complementary organismal functions could arise from differential regulatory
mutations [14,15]. Thus, if duplicate genes become expressed in different cell
types or developmental stages, they might become indispensable and resistant
to loss even if their associated peptides remain interchangeable. Through this
mechanism, novel spatiotemporal roles can emerge, with numerous individual
examples of cis- or trans-regulatory subfunctionalization known, for example, in
teleost fish .
The well-studied amphibian Xenopus laevis has chromosome number (2N = 36)
and genome size (~ 3 Gb), roughly double that of its congener Xenopus (formerly
Silurana) tropicalis (2N = 20, ~ 1.5 Gb) [16,17]. This difference is attributed to a
merger of two diploid progenitors originating ~ 40 million years ago [16,180-20].
Allotetraploidy is suggested by the ease with which modern Xenopus species can
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Hellsten, Uffe; Khokha, Mustafa K.; Grammar, Timothy C.; Harland,Richard M.; Richardson, Paul & Rokhsar, Daniel S. Accelerated Gene Evolution and Subfunctionalization in the Pseudotetraploid Frog Xenopus Laevis, article, March 1, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc886588/m1/5/: accessed May 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.