When Double is not Twice as Much

Abstract

Gene and genome duplications provide a playground for various selective pressures and contribute significantly to genome complexity. It is assumed that the genomes of all major eukaryotic lineages possess duplicated regions that result from gene and genome duplication. There is evidence that the model plant Arabidopsis has been subjected to at least three whole-genome duplication events over the last 150-200 million years. As a result, many cellular processes are governed by redundantly acting gene families. Plants pass through two distinct life phases with a haploid gametophytic alternating with a diploid sporophytic generation. This ontogenetic difference in gene copy number has important implications for the outcome of deleterious mutations, which are masked by the second gene copy in diploid systems but expressed in a dominant fashion in haploid organisms. As a consequence, maintaining the activity of duplicated genes might be particularly advantageous during the haploid gametophytic generation. Here, we describe the distinctive features associated with the alteration of generations and discuss how activity profiles of duplicated genes might get modulated in a life phase dependent fashion.

Keywords: alternation of generations; diploid; flowering plants; gene duplication; haploid.

Figure 1

Hypothetical model of life phase dependent redundancy adjustment. (A) The diploid sporophyte and the haploid gametophytes exhibit, by definition, different redundancies for a given locus. Deleterious mutations are consequently masked in the sporophyte but dominantly expressed in the gametophyte. (B) Gene duplication increases redundancy levels in the haploid and diploid phase. (C) If the activity of a given gene is preferentially maintained in the gametophyte, different redundancy levels between the sporophyte and the gametophytic life phase might be buffered. Black bar, ancient locus; green bar, duplicated locus, gray arrows indicate redundancy differences.

Figure 2

Expression patterns of three pre-mRNA splice factor encoding gene pairs in the ovule. Recent work in the authors’ laboratory indicate that AtBRR2, AtPRP8, and GFA1 are encoded by genes (At1g20960, At1g80070 and At1g06220, respectively) expressed in sporophytic and gametophytic tissue (A,C,E), but also by a gene copy, AtBRR2L (At2g42270), AtPRP8L (At4g38780), and GFL (At5g25230) that is preferentially active in the female gametophyte (B,D,F). The micropylar half of the female gametophyte is in focus and encircled by a dottet line.