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The mystery of the missing seed

April 10, 2020

    One morning as I was chopping up the different fruit for my smoothie.  I wondered why the bananas did not have seeds but the blueberries and strawberries did.  I was wondering about that and this is what I found out that bananas originally did not look like the bananas that we see and eat today because they contained huge seeds throughout.  Bananas were originally cultivated from Southeast Asia and the western pacific. There were two wild species, Musa acuminata(AA) and Musa balbisiana(BB), native to that area that derived the bananas we know and love today. 

     The Musa acuminate(AA) and Musa balbisiana(BB) are considered diploids.  These bananas were not preferred because the large stony seeds made eating them unpleasant.  Diploid bananas had two copies of each gene-bearing chromosome. Since the diploid banana was unpleasant to eat, farmers started to breed bananas with fewer seeds to evolve them.  They used parthenocarpy and seed-sterility. Parthenocarpy is the capacity of fruits to grow and become full edible fruits without pollination. This means that the fruits would not require to have seeds inside to reproduce.  They slowly got rid of the seeds in the plant by doing a combination of inefficient fertilization, early abortion of the seed after fertilization, and sterile gametes. These processes allowed for future crops to have fewer and fewer seeds in them.  Because bananas don’t have seeds they have lost the ability to reproduce by seeds.      

     Another way humans evolved the banana fruit was by cross-breeding different types.  They would crossbreed Musa accuminata with Musa balbisiana. When the farmer selected the bananas to crossbreed they picked distant relatives so that when crossed they would have mismatched chromones which make it difficult to produce fertile ovule to pollinate.  With the crossbreeding the triploid banana was created. Triploid bananas were productive, larger, hardier, and drought-tolerant plants. This is why they were chosen over their diploid counterparts. Farmers still need diploid bananas for breeding since they are the sources of resistance genes to biotic factors.  Because of all the cross-breeding, most of the bananas grown today has very little gene variety. This means that if there was a worldwide pest outbreak, it would wipe out a majority of the bananas because of how common their genes are to one another. Because of this, the banana industry has adopted chemical treatments to control and prevent pest outbreaks.  Therefore the banana industry has adopted chemical treatments to control it. The new breed extended the range of the plant’s character and quality features. This led to the new breed of bananas being able to grow and breed in seasonally drier climate zones. This lead to the banana expanding into West Africa. Because of this migration when Europeans visitors came to West Africa they took the banana plant with them to the New World.  

 

References

Perrier, X., Bakry, F., Carreel, F., Jenny, C., Horry, J. P., Lebot, V., & Hippolyte, I. (2009). Combining biological approaches to shed light on the evolution of edible bananas. Ethnobotany Research and Applications, 7, 199-216. https://agritrop.cirad.fr/550538/1/document_550538.pdf 

Simmonds, N. W. (1987). Classification and breeding of bananas. Banana and plantain breeding strategies, 69-73. file:///C:/Users/lelae/Downloads/PR021.pdf 

Baker, H. G. (1972). Human influences on plant evolution. Economic Botany, 26(1), 32-43. https://www.jstor.org/stable/pdf/4253307.pdf 

All About Bananas: Producers, Where They’re Grown & Why They Matter. (n.d.). Retrieved February 7, 2020, from https://www.bananalink.org.uk/all-about-bananas

Creste, S., Neto, A. T., Vencovsky, R., Silva, S. D. O., & Figueira, A. (2004). Genetic diversity of Musa diploid and triploid accessions from the Brazilian banana breeding program estimated by microsatellite markers. Genetic Resources and Crop Evolution, 51(7), 723–733. Doi: 10.1023/b:gres.0000034578.37951.c4

Castillo, C., & Fuller, D. Q. (2016). Bananas. The Oxford Handbook of the Archaeology of Diet. doi: 10.1093/oxfordhb/9780199694013.013.7

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.