Move over Mendel’s peas, there’s a new plant in town (at least for St. Patrick’s Day). Trifolium repens, or white clover, is a widely cultivated member of the bean family. Originally native to Europe and parts of Central Asia, it now graces meadows, lawns, pastures, and gardens worldwide. Its high protein content and nitrogen-fixing abilities make it a popular addition to forage grass mixes. Moreover, its low height and tolerance of mowing have made it a staple in lawn and garden ground cover mixes. Beyond its utilitarian uses, white clover also plays a crucial role in supporting pollinators like honeybees. Yet amidst its practical virtues, there’s one enchanting aspect that steals the spotlight: the elusive promise of good fortune embodied by its four-leafed variant.
But here’s the kicker: four-leaf clovers aren’t exactly what they seem. Those “leaves” you see? They’re actually leaflets—tiny leaf-like parts—clustered together on a single stem. Think of it like a leafy bouquet, but on a smaller scale. These leaflets don’t grow independently from the main plant stem; instead, they sprout from a central point on the leaf’s stalk, known as the petiole.
Now, the odds of stumbling upon one of these elusive four-leaf clovers? A one in 10,000 chance. Sounds like it’d be pretty hard, right? Well, given the sheer size and density of clover patches, statistically, there should be one in roughly every 13×13 square foot area. But scientists and breeders are pulling out all the stops to tip the odds in our favor. Back in 2008, a team of researchers decided to play bombarding clover plants with radiation in a bid to create new varieties. One resulting variety named “Jeju Lucky-1” emerged from the chaos boasts a nifty 60-40 mix of four-leaf to three-leaf clovers. Talk about making luck more accessible!
A famed plant breeder in Japan took a different tack to ramping up clover luck. Shigeo Obara spent a lifetime crossbreeding and creating different clover varieties. After countless rounds of clover matchmaking, he managed to cultivate a super-clover with a whopping 56 leaflets.
Another clover enthusiast is Dr. Wayne Parrott, a plant geneticist at the University of Georgia. While much of his work focuses on the genetics of soybeans and several biofuel grasses, he is also passionate about clovers. (Just watch this video!). Dr. Parrott and his team have been searching for the genetic control of key traits in clover – including leaflet number.
The work isn’t easy! For one thing, white clover is an allotetraploid. This means that it (ironically enough) has four sets of chromosomes. Such polyploidy is fairly common in the plant kingdom. Allotetraploids occur through hybridization, where two distinct species interbreed, resulting in offspring with a doubled chromosome number compared to their parents. Scientists studying the phylogeny (family tree) of clover think that the two different parental species that formed white clover may be T. nigrescens, T. occidentale, T. pallescens, T. uniflorum, or an extinct/yet to be discovered clover species.
Studying inheritance patterns in polyploids is complex. The multiple sets of hom*ologous chromosomes make it difficult to accurately determine allelic relationships and inheritance patterns. Additionally, polyploids may exhibit gene redundancy and dosage effects, where multiple copies of genes can interact in complex ways, impacting phenotypic traits and gene expression. In the case of four leaf clover, there was also another challenge – whether a plant had three or four leaflets was also influenced by the environment.
Nevertheless, Parrott and his team successfully identified the genetic basis of the four-leaf clover using a combination of modern molecular-based genetics tools and traditional breeding techniques. By cultivating two distinct plant populations in different environments and analyzing DNA extracted from them, they were able to map the areas that controlled leaflet number. And here they did have a bit of luck. In a species known to have complex genetics where many genes interact together to affect things like leaf color and shape, leaflet number was traced to a single gene.
If your classroom’s ready for a little four leaflet luck this semester consider introducing biotechnologies though the primary lit on the clover (long list below). And don’t forgot to check out our resources on reading primary literature and our list of other great biotech papers.
Williams, Warren M.; Ellison, Nicholas W.; Ansari, Helal A.; Verry, Isabelle M.; Hussain, S. Wajid (2012-04-24). “Experimental evidence for the ancestry of allotetraploid Trifolium repens and creation of synthetic forms with value for plant breeding”. BMC Plant Biology. 12: 55. doi:10.1186/1471-2229-12-55. ISSN1471-2229. PMC3443075. PMID22530692.
Williams, W. M.; Ansari, H. A.; Hussain, S. W.; Ellison, N. W.; Williamson, M. L.; Verry, I. M. (2008-01-01). “Hybridization and Introgression between Two Diploid Wild Relatives of White Clover, Trifolium nigrescens Viv. and T. occidentale Coombe”. Crop Science. 48 (1): 139–148. doi:10.2135/cropsci2007.05.0295. ISSN1435-0653.
Badr, A.; El-Shazly, H. H.; Mekki, L. (2012-06-01). “Genetic diversity in white clover and its progenitors as revealed by DNA fingerprinting”. Biologia Plantarum. 56 (2): 283–291. doi:10.1007/s10535-012-0088-0. ISSN0006-3134. S2CID14983555.
Hand, Melanie L.; Ponting, Rebecca C.; Drayton, Michelle C.; Lawless, Kahlil A.; Cogan, Noel O. I.; Brummer, E. Charles; Sawbridge, Timothy I.; Spangenberg, German C.; Smith, Kevin F. (2008-10-01). “Identification of hom*ologous, hom*oeologous and paralogous sequence variants in an outbreeding allopolyploid species based on comparison with progenitor taxa”. Molecular Genetics and Genomics. 280 (4): 293–304. doi:10.1007/s00438-008-0365-y. ISSN1617-4615. PMID18642031. S2CID24487483.
Atwood SS, Hill HD. (1940). The regularity of meiosis in microsporocytes of Trifolium repens. American Journal of Botany 27: 730-735. link
Pustahija F, Brown SC, Bogunic F, Bašic N, Muratovic E, Ollier S, Hidalgo O, Bourge M, Stevanovic V, Sijak-Yakovev S. (2013). Small genomes dominate in plants growing on serpentine soils in West Balkans, an exhaustive study of 8 habitats covering 308 taxa. Plant and Soil 373: 427-453.
Griffiths AG, Barrett BA, Simon D et al. (2013). An integrated genetic linkage map for white clover (Trifolium repens L.) with alignment to Medicago. BMC Genomics 14: 388.
Zhang Y, Sledge MK, Bouton JH. (2007). Genome mapping of white clover (Trifolium repens L.) and comparative analysis within the Trifolieae using cross-species SSR markers. Theoretical and Applied Genetics 114: 1367-1378.
Griffiths AG, Moraga R, Tausen M et al. (2019). Breaking Free: The genomics of allopolyploidy-facilitated niche expansion in white clover. The Plant Cell 31: 1466-1487.
Sveinsson S, Cronk Q. (2014). Evolutionary origin of highly repetitive plastid genomes within the clover genus (Trifolium). BMC Evolutionary Biology 14: 228.
Griffiths AG, Moraga R, Tausen M et al. (2019). Breaking Free: The genomics of allopolyploidy-facilitated niche expansion in white clover. The Plant Cell 31: 1466-1487.
Zhang H, Tian H, Chen M et al. (2018). Transcriptome analysis reveals potential genes involved in flower pigmentation in a red-flowered mutant of white clover (Trifolium repens L.). Genomics 110: 191-200.
