Have you ever wondered how plants avoid self-pollination? Nuts have evolved to prevent this by timing the flowering of male and female flowers. The walnut flowers of a certain sex bloom first, being followed, after a while, by the others.
Nuts can be different depending on the sex of the flowers that bloom first, either female or male, and we’ve known this since 1877 when Charles Darwin observed this phenomenon. This pattern has been noted in both cultivated walnuts and their wild relatives, among which the ratio of trees starting with female flowers to those starting with male flowers is about 1:1.
Only in the 1980s was it discovered that this phenomenon is determined by a single genetic locus, identified by Scott Gleeson, a student at UC Davis (USA). However, there was still much to understand about how walnut flowers work.
How do walnut flowers work?
“Walnuts and pecans exhibit temporal dimorphism, alternating between male and female flowering throughout the season. This phenomenon has been known since the 19th century, but until now it has not been understood at the molecular level.” Jeff Groh saidstudent in population biology at UC Davis and lead author of a new study.
The study by Groh and his colleagues looked in detail at the determination of sex in the tree family Juglandaceaewhich includes walnuts, hickory and pecans. With the support of Professor Graham Coop of the Department of Evolution and Ecology, the researchers collected data from UC Davis’ walnut breeding program and California black walnuts on campus.
The scientists determined which trees started with female flowers and which with male flowers, then sequenced their genomes to identify patterns associated with this character. The results showed that in walnuts, there are two variants of the gene identified by Gleeson that influence the order in which the flowers bloom. These variants have been observed in at least nine nut species, and the system has been found to be stable for about 40 million years, notes IFL Science.
Trees that resemble mammals more than we thought
The study also looked at pecans, finding that they too have a flowering strategy controlled by a distinct genetic region, but located in a different part of the genome than walnuts. For pecans, this mechanism evolved about 10 million years earlier and may represent an example of convergent evolution, where a common ancestor initiated the trend and the exact genetic mechanism gradually changed.
Moreover, the system appears to maintain a 1:1 balance between trees starting with female flowers and those starting with male flowers. When one type becomes more common, the other gains a reproductive advantage, restoring the balance. This phenomenon is similar to how sex chromosomes work in animals.
“It is quite atypical to maintain variation over such a long period, and there is a clear parallel with a common mode of sex determination,” Groh explained.
The system that maintains the 50:50 balance between female-flowered and male-flowered nuts is similar to how the X and Y chromosomes in humans (and other mammals) are kept in balance by the influence that an excess of one has on reproductive potential of the other.
The study was published in magazine Science.
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Source: www.descopera.ro
