Agriculture Reference
In-Depth Information
duplication of a haploid genome by either endoreduplication, or more
likely cell fusion, was probably selected by an intolerably high rate of
mutation, caused by perhaps an environmental condition, for example,
high UV irradiance, or by invasive DNA, or both.
Genomes of many organisms can be composed of very large amounts
of transposable elements (TE) or sequences derived from them, often
composing 90% or more of the genome. Class I TEs such as retrotrans-
poson sequences represent by far the largest amount of invasive repeti-
tive DNA found in genomes (Bennetzen and Kellogg 1997; Brunner et al.
2005; Lisch and Bennetzen 2011), probably because they do not excise
and have the ability to move by replication via a reverse transcriptase-
mediated RNA intermediate, followed by insertion into new sites via an
integrase. This allows them to multiply and spread exponentially in the
invaded genome using a nucleotide pairing information system as used
in DNA replication and the Central Dogma process (Bennetzen and
Kellogg 1997; Bennetzen and Zhu 2011). This TE replication represents
the initiation of several processes by which nucleotide pairing conveys
information transfer without Central Dogma involvement. At the initia-
tion of invasive events by TE, almost all of the insertions would be in
genic regions since the TE intergenic repetitive sequences would not
have been established. Buffering of such an extreme amount of mostly
deleterious insertion mutations by creating a diploid genome, most
likely by a primitive cell fusion event, would then encourage selection
for quality check mechanisms of the genome, that is, a return to haploidy
by a balanced genome reduction (Walbot and Evans 2003). This can
occur by pairing of homologous linkage groups (chromosomes) followed
by reduction division in meiosis. Natural selection would remove or
reduce from the resulting haploid gametophytic generation population
any individuals with severe deleterious genomic changes (Walbot 1985).
This would be followed by a return to the buffered diploid state by
random fusion of gametes produced from members of the surviving
haploid population. Many haploid individuals with deleterious TE
insertions in their genomes would also lose new bene
cial alleles by
this process. This would result in strong selection pressure for evolution
of homologous nucleotide pairing to then establish some mechanism
such as a crossing over recombination system that would allow for
partial DNA removal that more selectively rejects the deleterious
unpaired invaded regions. In other words, independent assortment of
recombined paired homologous genomes into gametes, and their subse-
quent rejoining by fusion into diploid genomes would allow selection for
diploid combinations of linkage groups that have had disproportionate
amounts of only the damaging invasive DNA regions removed.
