Avocado Flower Mutations

Avocado flower ‘petals’ are comprised of 2 whorls, each of three similar-sized petal-like structures called tepals. No one can decide if avocado ‘petals’ are actually sepals or really sepal-like petals – thus the compromise name ‘tepal’.

According to a University of California avocado expert, in some small-fruited species in the genus Persea the subtending tepal whorl is quite a bit smaller than the overlaying whorl. As a result, the flowers in these species look like they have 3, not 6 tepals.

But the avocado almost always has 6.

On 9th November 2018, towards the end of the avocado flowering (for most cultivars) I noticed a few flowers in a Hashimoto tree that had one whorl so greatly reduced in size that at first glance I mistook it for a three-tepal abnormality.

Checking other cultivars, I found a few flowers with 4 tepals. These were mainly in Hass seedlings, Pinkerton, and Gwen. Today, a year later, I noticed several 4 tepaled flowers in a Maluma tree.

In all cases, the 4 – tepaled flowers have a single whorl. There are no subtended tepals of any size.

Several other Maluma flowers have 8 tepals, as does a seedling expressing many characters typical of the Mexican ecotype.

I haven’t seen an explanation of these phenomena in avocado.

In the case of the reduced whorl resulting in an apparent 3 – tepal flower, maybe a point mutation resulting in a loss-of-function meristem mutation (ptl) in PETAL LOSS (PTL) gene or affectors as discussed by Quon et al 2017 in Arabidopsis?

https://www.frontiersin.org/articles/10.3389/fpls.2017.00152/full

The 4 tepal flower could perhaps be the result of a different gene action. Quon et al 2017 cite Lampugnani et al., 2013 to note that “growth-induced distortions to auxin accumulation internal to the sepal whorl” may disrupt petal initiation. Several genes control auxin transport and accumulation and when these are compromised auxin levels in meristem tissues may be abnormally low. Assuming, of course, that Persea has homologous genes.

The AUXIN1 gene (AUX1) has variants in Arabidopsis whose partial- loss-of-function mutant form arose as a point mutation (e.g. aux1-7). If there is an homolog in avocado, and a point mutation arose (the most likely explanation), it might result in sepal loss. The loss of 2 tepals in a whorl may result in the creation of a 4 tepal whorl.

The 8 tepal flower is a bit more difficult to explain.