One plays the good guy, the other plays the bad guy, what's the matter with the "Yin Yang Pomelo"?

Have you ever seen this kind of Yin-Yang grapefruit ? Oh, one is playing the good cop, the other is playing the bad cop, you have played all the roles, how can it not be considered cheating!

Yin Yang Pomelo｜Xiaohongshu: @莲

But they really didn't cheat, because the cheating (hybridization) grapefruit would not be able to grow like this: only plants with the "mosaic dominant" genetic trait can grow different-colored fruits after hybridization, but the citrus genus does not have this genetic trait, no matter how much hybridization, the flesh will only be the same color .

For citrus plants to grow like this, they either rely on arranged marriages (heterologous chimeras obtained through grafting, etc.) or sudden self-expression (homologous chimeras obtained through mutation, etc.). Which one is the real situation can only be known through sequencing. However, given that the natural mutation rate of citrus plants is so high , we tend to think that this is more likely to be a chimera phenomenon caused by mutation.

What is a chimera?

The word "chimera" is not difficult to understand literally, and its English name chimera is more vivid. This name comes from the Chimera , a monster with a lion's head, a goat's body, and a snake's tail in ancient Greek mythology. Today, in biology, chimera refers to the phenomenon that the same organism has multiple sets of genetic material at the same time, thus showing a variety of different traits .

Chimera in mythology | Lampas Group / wiki commons

The earliest chimera was discovered by Italian horticulturist Pietro Nati. In 1640, in a villa in Florence, he found a strange citrus fruit growing from the grafted joint of Florentine citron and lime. It looked like a little monster that had its parent split in half and glued together with super glue. This example also reflects the unshakable status of citrus fruits as the elders in the chimera world!

The offspring of an arranged marriage between a citron and a lime is called bizzaria, which is pretty bizarre indeed… | Labrina / Wikimedia Commons

Nowadays, it is not surprising to see all kinds of strange chimeras in the plant world. For example, a branch can have both red and white flowers, or even red and white "Golden Peach":

Golden peach in Gongqing Forest Park | Shanghai Forestry Bureau

Another example is the chimera apple:

The chimera in the middle seems to be a combination of the left and right sides | growingproduce.com

Chimera grapes:

Will it be half sour and half sweet? | glossary.wein.plus

And the chimera cucumber:

Leaves of ... | Shubin Saha / University of Kentucky

There are also chimeric peppers, chimeric green peppers, chimeric tomatoes, chimeric avocados...

Various "Chimeras" | Plantpathologycy-Agriculturecy / facebook

Reality is more confusing than "cheating "

The genus Citrus is a large family in the world of chimeras, and old fans of the calendar must have eaten their melons a long time ago.

The "yin and yang" grapefruit mentioned at the beginning of this article is also a member of the genus Citrus. The confusion of the genus Citrus is due to the fact that pollination between species is very easy, leading to promiscuity (feeding), and on the other hand, it is due to the highly heterozygous genetic characteristics caused by the extremely high mutation rate . Here is a comparison: the probability of natural occurrence of citrus chimeras is 0.02% to 3.97%, while the natural mutation rate of bananas is only 0.00002%.

Download progress 80% grapefruit | reddit.com

Can I grow a "yin and yang" pomelo tree?

You may ask, if I keep the seeds of this grapefruit, can I grow more Yin Yang grapefruits? The answer is... yes, but it cannot be "grown".

The flesh of this grapefruit has mutated, but whether the seeds inside the flesh have also mutated depends on the stage and part of the body where the mutation occurs. Natural mutations actually occur all the time in organisms, but if they occur in differentiated organs, the impact on the plant as a whole is very limited. Only when mutations occur in cells with differentiation ability (such as meristems) can chimeras be formed.

The tunica-corpus theory holds that the plant's stem apical meristem is divided into three layers, which are labeled L1, L2, and L3 from the outside to the inside; the three layers of cells divide in different ways, and mutations occurring in one of the layers of cells will eventually appear on the plant body in different mosaic forms.

Mutated cells form chimeras in different forms, eventually forming periclinal chimeras, sectorial chimeras, and mericlinal chimeras. References [3]

The trouble is that the flesh of the citrus fruits we eat is usually made of epidermal hair cells differentiated from the L1 layer , but the seeds are mostly differentiated from the L2 layer .

The pigments in the red flesh of grapefruit are generally the accumulation of lycopene, carotenoids or anthocyanins. For white grapefruit, if the gene regulating pigment accumulation in the L1 layer mutates , red grapefruit cells will appear locally: if the red grapefruit cells are domineering enough, they can occupy the entire epidermis and become a whole red grapefruit, forming a "peripheral chimera" (the first case in the diagram above); if the red grapefruit cells are weak and cannot completely destroy the white grapefruit cells, a half-red and half-white "peripheral differentiation chimera" will be formed (the third case in the diagram above). However, whether it is a pure red grapefruit or a half-red and half-white grapefruit, the chimeras in these two cases cannot affect the genetic information of the seeds , after all, the mutation of the L1 layer has no effect on the seeds. Therefore, even if the seeds in this grapefruit are collected and planted, only all-white grapefruits can grow, and the yin-yang grapefruit tree cannot be obtained.

If the seeds are to mutate, the mutation must enter the L2 layer. Originally, grapefruit cells can only undergo periclinal division (that is, they continue to expand along the direction of the rings in the same layer), but if the grapefruit cells are sufficiently rebellious and suddenly change direction and begin periclinal division (dividing outward or inward along the direction perpendicular to the rings), the grapefruit cells can enter the L2 and L3 layers , forming a "fan-shaped chimera" (the second case in the diagram above).

"Fan-shaped chimeras" may affect the genetic information of reproductive cells , but it does not mean that the seeds it produces will show this variation. Specifically, the tree that produces yin-yang pomelo may indeed produce pollen and female gametophytes with variant genetic information; but the essence of chimeras is that the plant has two sets of genetic information at the same time, and they are expressed at the same time. Seeds produced by sexual reproduction can only have one set of genetic information (not two sets) even if the parents are chimeras. Therefore, chimera parents still cannot give birth to yin-yang children.

Bizarre citrus mutant fan-shaped chimeras | References [1]

Fortunately, although sexual reproduction is not feasible, horticulturists have their own way. For the tree that produced this Yin-Yang pomelo, even if the mutation only occurred in the L1 layer, horticulturists can still preserve this mutation through vegetative propagation methods such as cuttings . In fact, the selection and breeding of many citrus plant varieties are completed through chimera bud mutation selection .

So, don’t dream of growing your own Yin-Yang pomelo tree. Instead, let’s imagine what would happen if someday a breeder takes a fancy to this trait!

References

[1] Zhang Min (2006) Isolation of citrus fruit fan-shaped chimeras and genetic studies of two groups of grafted chimeras [PhD dissertation]. Wuhan: Huazhong Agricultural University

[2] Li Tianfei et al. (2002) Journal of Hubei University: Natural Science Edition. 24 (1): 6

[3] Srivastava LM (2002) Plant growth and development: hormones and environment. Elsevier

[4] Ohtsu et al. (1994) Jpn. J. Phytopathol. 60 (1):20-26

Author: Garlic

Editor: trombonist

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