The shamrock is one of the classic symbols of St. Patrick’s Day. Shamrocks, or clover, are generally found with three symmetrical leaves. However, the luckiest among us will occasionally stumble upon an exceedingly rare four-leaf clover. In fact, it’s estimated that only 1 out of every 5,000 clover plants will grow a fourth leaf. Lucky indeed!
So why do the vast majority of clover only have three leaves? It all comes down to genetics. Or maybe it has to do with random mutations that arise during early leaf development. Environmental conditions during growth? It turns out that scientists are not actually sure what causes a four-leafed clover to sprout amidst thousands of the standard three-leafed variety. What is known is that it’s a lot more complex than you would imagine.
Plant genetics can be significantly more complex than you would initially imagine. For example, the standard white clover (Trifolium repens) is tetraploid, containing four complete copies of the genome in each cell. To make it even more complicated, white clover are allotetraploid – they contain genomes from two different clovers (T. occidentale and T. pallescens) that hybridized thousands of years ago. The resulting clover seems to be better adapted to a broad range of environments and has thrived, spreading across Europe, Asia, and North America, although white clover is now also found worldwide.
The complex genetics don’t stop there. In the wild, clover plants are almost incapable of breeding with themselves, instead requiring outbreeding with other plants. This makes it tricky to determine where each chromosome originates. Combined with the increased number of chromosomes (remember, they are tetraploid) this makes inheritance studies extremely difficult. This has not stopped plant geneticists from trying, and several studies have made discoveries that shed light on the origin of the four-leaf phenotype.
Scientists at the University of Georgia published a study in 2010 that uncovered part of the puzzle. They started by growing populations of clover in summer and winter conditions and then mapping the genetics of the three and four-leafed variants. This analysis uncovered regions of the genome that seem to influence the number of leaves being formed. Curiously, the experiment found regions on two different chromosomes that seem to be responsible. In addition, the warmer summer conditions resulted in more than double the number of four-leafed plants.
Although the science is still unsettled many people are interested in figuring it out. Farmers could grow clovers with more leaves to provide additional food for livestock and souvenir vendors would have an easier time harvesting four-leaf clovers to sell in keepsakes. So, what should you do if you’d like to find a four-leaf clover of your own? Well, the science tells us that they are more likely in warmer summer months. Conveniently, clover grows more rapidly in the summer, giving you more plants to search through. And search you will… but don’t give up, there’s probably a lucky clover somewhere in the field!