During the analysis of the observations from a complete diallel cross of cacao clones at CATIE in Costa Rica it turned out to be impossible to determine for a number of traits their inheritance at the diploid level whereas this was possible using tetraploidy as the prevailing ploidy level. In correspondence with Dr. Allan in Ecuador during the 1980ies this view was supported. The 'discovery' of the publication of Opeke and Jacob (1967) in which the formation of quadrivalents in 36.6% of pollen mother cells of the Nanay clone and the fact that this clone can be regarded primarily as an autotretraploid did make me think of using tetraploid inheritance to explain the genetics of a number of traits. This worked and makes me think that cacao is a relatively 'primitive' crop that is 'in transition' from a tetraploid into a diploid species. For more details see my thesis 'The systematic description of cacao clones and its significance for taxonomy and plant breeding (Engels, 1986).
The suggestion of tetraploid rather than diploid inheritance might be one of the (main?) reasons why breeding new cacao varieties with specific traits is so difficult!?
You have rightly mentioned that to facilitate breeding of new varieties it is of critical importance to understand the genetics of cacao better. You also mentioned that cacao is still behaving like a tetraploid, slowly evolving into a diploid. It is very interesting to know these facts
Thank you Suresh, your response is much appreciated! In case you know of 'similar situations' (i.e. a 'co-existence' of tetraploid and diploid forms of plants from one and the same crop species) and/or any observations you have made or heard of with respect to the difficulty to breed cacao, please share such information!
I believe Guayule (Parthenium argentatum Gray) falls into this realm.
Dear David,
Could you suggest any publication on this species? Would be useful and pertinent to see the details!
Thanks,
Jan
Why do you think that it is transitioning to a diploid? Medicago sativa is a successful autotetraploid, Tritcum aestivum is a successful allohexaploid (only behaves as a diploid) and many perennials have a more complicated genetic make-up.
You might be very right with your comment regarding autotetraploid or even allohexaploid species, but cacao has been regarded widely as a diploid species and consequently my thought that this is possibly the 'direction' of the species evolution. Most of the meiotic pairings in the pollen mother cells are bivalent, thus I was wondering if that particular clone 'Nanay' (which could theoretically be a selected tree from the wild!? JE) is an exception or that the phenomenon of a significant percentage of quadrivalents has also been observed in other clones?
thanks Johannes, we have no cacao here. Are they reproduced vegetatively for production? Is seed set and product quality impacted by this meiotic behavior?
Thank you Douglas, good questions! Honestly speaking, when I was doing my PhD on cacao genetic resources in Costa Rica I was not aware of this issue and had not observed the Nanay clone more closely in the CATIE collection. The issue on ploidy level only came up when I was trying to analyze the data of the diallel cross in Ethiopia, where three cacao trees were reported to exist!? Indeed, quite a number of the cacao varieties are actually clones.