Selfish genetic element confers non-Mendelian inheritance in rice

As these GEITP pages have previously discussed, the ~3-billion base-pair (bp) genome of human comprises only ~1% of the DNA that is actually responsible for functional protein-coding genes (this portion, ~30 million bases, is called the “exome”). [Therefore, whole-genome sequencing (WGS; also called next-generation sequencing, NGS) looks at DNA sequence of virtually all of the ~3 billion bp, whereas whole-exome sequencing (WES) examines only those 30 million bp.] About half of the remaining ~99% DNA consists of transposable elements (called “selfish DNA”) which are being discovered to have important functions, whereas the other half of the DNA has no known function (so far) and is called “junk DNA.” Richard Dawkins in his 1976 book first described “the selfish gene”, and then Leslie Orgel and Francis Crick’s 1980 Nature article described “selfish DNA.” More or less similar proportions (gene-selfish-junk DNA) are known to exist in other animals –– as well as plants.

The attached report deals with selfish genetic elements (SGEs) in rice. Accumulating evidence suggests that SGEs, which are DNA sequences that gain a transmission advantage relative to the rest of the genome, could drive genome evolution by causing hybrid incompatibilities and segregation distortion (i.e. deviation from the expected Mendelian ratio, half from one parent, half from the other parent) in different organisms; however, the role of SGEs in genome evolution and their underlying molecular mechanisms remains controversial. Authors herein demonstrate that qHMS7 –– a major quantitative genetic locus for hybrid male sterility between wild rice (Oryza meridionalis) and Asian cultivated rice (Oryza sativa) –– contains two tightly linked genes [called Open Reading Frame 2 (ORF2) and ORF3].

ORF2 encodes a toxic genetic elephant that aborts pollen in a sporophytic (in the life cycle of plants, such as rice, that have alternating generations, the asexual and usually diploid [pairs of chromosomes] phase, produces spores –– from which the gametophyte arises) manner, whereas ORF3 encodes an antidote that protects pollen in a gametophytic (in the life cycle of plants with alternating generations, the gamete-producing and usually haploid [single chromosomes] phase, produce the zygote –– from which the sporophyte arises) manner. Pollens lacking ORF3 are selectively eliminated, leading to segregation distortion in the progeny. Evolutionary analysis of the genetic sequence suggests that ORF3 arose first, followed by gradual functionalization of ORF2. Furthermore, this toxin-antidote system (ORF3) may have promoted the differentiation, and/or may have maintained the genome stability, in both wild rice and cultivated rice.

Science 8 June 2o18; 360: 1130–1132

ORF2 encodes a toxic genetic elephant
That’s funny.

Thank you for detecting this (deliberate) typo. You are the Winner this time. 😉 I was just checking to see if anyone was reading this (rather dry, complicated) plant genetics email carefully. Usually C. V. is the one who first detects my typos. Of course, it is meant to read “toxic genetic elements.”

An afterthought: I should have added a comment at the end of this quick review –– that, in the near future, we anticipate more clinical phenotypes (disorders) will soon be detected as being regulated/influenced by “selfish DNA.” These transposons do crazy, unexpected things.

COMMENT: ha ha… i thought it was a deliberate joke, you know like a “white elephant”. still cute.

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