Accidents happen all the time and sometimes living cells fail to divide properly. During cell division the genetic material that was supposed to be distributed equally between two cells may all stay in one of the cells. If this happens in body cells a cancerous tumor may develop or the cell may die. If it happens in an embryo at the very early stages of development at rare occasions these individuals may survive to maturity and may even reproduce to start a new species. Evolution by increasing genomic content by doubling or tripleing or quadrupling… is called polyploidization.
Before getting into esoteric concepts for explanation of chromosome reductions during plant evolution you may need a refresher of some basic molecular biological descriptions and definitions. The following lecture from the Khan Academy provides an excellent summary on chromosome terminology:
Polyploidization very rarely happens in animals who have a rather strict developmental body plan. In plants polyploidization is more frequent.
Repeated polyploidization, with whole genome duplicated or triplicated, increases chromosome numbers in plants. However, plant chromosome numbers have usually been restored to a narrow range. A comparison of banana and rice genomes showed that their common ancestor had 2n = 14 chromosomes (n=7 basic chromosomes). The ancestral plant that evolved into banana and rice doubled its chromosomes to 28. Throughout the evolution the ancestral plant had some of it’s chromosomes fused and eventually the chromosome number decreased to 24 which is now preserved in rice and some other grasses.
Chromosome number reduction continued in other major grass lineages, at last Brachypodium, barley, foxtail millet, and sorghum have 2n=10, 14, 18, and 20 chromosomes nowadays respectively. Maize have 20 chromosomes, the same basal number as sorghum. Nonetheless, maize was subjected by another tetraploidization, and chromosome number doubled to 40, and then reduced to 20. Wheat has 42 chromosomes after subjected to a hexaploidization, merging 3 different sub-genomes into one. In Arabidopsis, 8 ancestral chromosomes were reduced in A. thaliana but not in A. lyrata.
Chromosomes fuse into each other but how? What mechanism is governing chromosome number reduction? Recently, Dr. Xiyin Wang from University of Georgia and Hebei United University, together with colleagues, proposed a theory to explain this mystery. The study presents a telomere-centric model of chromosomal crossovers.
Telomeres are structures at two ends of a chromosome, and are used to preserve its integrity; a centromere is a structure often in the middle part of a chromosome, and helps separate two pairing homologous chromosomes. Newly formed chromosomes utilize the existing telomeres of ‘invaded’ and centromeres of ‘invading’ chromosomes, other centromeres and telomeres become lost. The following video provides a good information basis about animal telomeres and is also true for plants as well:
During chromosome rearrangement, satellite chromosomes containing two telomeres form tiny DNA but they were often lost. For a chromosome to survive it must have both telomeres and centromeres together on their proper locations.
The loss of satellite chromosomes resulted in chromosome size reduction. The hypothesis proposed in plants can also explain chromosome number reduction in mammals (including human) and yeast, possibly being a general mechanism of restoring small linear chromosome numbers in eukaryotes.
For a detailed description of the chromosome number reduction hypothesis, see Wang et al:
Telomere-centric genome repatterning determines recurring chromosome number reductions during the evolution of eukaryotes. New phytologist. 2014. DOI: 10.1111/nph.12985