Monday, March 16, 2015
I am often asked why Sunshine Vetiver (Chrysopogon zizanioides - south Indian variety) and other similar vetiver cultivars are sterile? Needless to say it's all about genetics and the number of chromosomes. I hope the following will help in understanding the why and how!:
Sunshine vetiver (named after its US location - Sunshine, Louisiana) is a triploid (3n). Most plants and animals are diploids (2n) (2 sets of chromosomes). A triploid has an extra set, and because of this is sterile. For example triploid water melons (seedless) and triploid carp are both sterile and cannot reproduce themselves. Sunshine Vetiver is the same. Us humans are diploid and can only too well reproduce ourselves!
These quotes may help you understand this:
"Triploids. Sometimes "errors" occur during cell division. If the chromosomes fail to segregate properly to the daughter cells, the result can be a diploid gamete (with two sets of chromosomes rather than the usual one). If this gamete unites with a normal, haploid gamete, then the resulting zygote will be triploid—that is, it will have three sets of chromosomes. Though this condition can sometimes result in plants with larger flowers or other desirable characteristics, triploid plants are usually sterile. Because the chromosomes occur in threes, rather than in pairs, the chromosomes cannot segregate evenly to the daughter cells. This often results in infertile pollen and reduced sterility in the female plant.
Despite their sterility, triploids are still horticulturally important. Not only are triploid flowers often larger and more colorful than their diploid counterparts, since they don’t produce mature seeds the flowers often last longer as well, and require little or no "dead-heading." One interesting triploid is the familiar banana. Diploid bananas do exist, but they have hard seeds. However in triploid bananas, the seeds abort, leaving only the tiny remnants you see. So don’t bother looking for banana seeds—at least not if you want to eat the fruit! The plant must be multiplied from cuttings".
Another example: "seedless water melon cultivars are produced by crossing a tetraploid (4X=44) inbred line as the female parent with a diploid (2X=22) inbred line as the male parent of the hybrid. The reciprocal cross (diploid female parent) does not produce seeds. The hybrid is a triploid (3X=33), and is female and male sterile. Triploid plants have three sets of chromosomes, and three sets cannot be divided evenly when they go into two daughter cells during meiosis (the cell division process that produces the gametes). Since the triploid hybrid is female sterile, the fruit are seedless. Because the triploid is also male sterile, it is necessary to plant a diploid cultivar in the production field to provide the pollen that stimulates fruit to form. Usually, one third of the plants in the field are diploid and two thirds are triploid. Cultivars should be chosen that can be distinguished easily so the seeded diploid fruit can be separated from the seedless triploid fruit for marketing.”
Here are some links to papers that may help you with regard to vetiver and its sterility and potential breeding:
Hopkinson (Australia) tested 40,000 Monto vetiver seeds and showed that all but three were sterile. On the basis of this research the Australians allowed Monto to be distributed to the public
USDA/NRCS - Robert Joy (Hawaii) confirmed that Sunshine (triploid) is sterile and allowed its general release for use in Hawaii.
PACIFIC ISLAND ECOSYSTEM AT RISK (PIER) rating showed Vetiver (Sunshine equivalent) at minus 9 level of invasiveness - where as the north Indian fertile diploid vetiver is a plus 9 on the invasive list. Hawaii and Australia allow plants with a plus1 or less to be imported. Sunshine vetiver at -9 is very safe.
USA (Robert Adams) DNA tested vetiver from around the world. The sterile cultivars were shown by DNA comparison to be virtually genetically the same. Confirming that Monto Vetiver (Australia) = Sunshine Vetiver (USA)
Lavania (India): Umesh Lavania is one of the worlds leading breeders of vetiver. His papers give you a closer look at the science and differentiation between, diploid, triploid, polyploid vetivers and how he is using this to breed potentially new vetivers that may work better for different applications and yet at the same time remain a sterile plant.
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