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As we can see on this larch, male and female cones are sometimes directed to grow on the opposite side of the branch. We were told in school that this mechanism works against self-fertilization. Also they flower in different times, so they are mostly fertilized from the other conifers.

When I saw this picture, I got that idea, that maybe they face the branches on above by design: To decrease the genetic uniformity at least a little.

But how important this effect actually is? And how big is the genetical difference between two branches? Are there any other factors taking effect in the tilt of the cones?

larch

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I don't understand your hypothesis. But the number of pairwise differences between two cells assuming neutrality ($\pi$) in the two different branches in $\pi = 2 \mu L t$, where $\mu$ is the somatic nucleotide mutation rate (around $10^{-11}$ on average), $L$ is the length of the sequence of interest (eventually the whole genome). $L$ can therefore be of the order of $10^9$. $t$ is the number of cell generation since the two cell lineages split. $t$ depends on the relative positions of the two branches but it could be $10^4$ cell generations. With these very quick approximations, we can estimate the number of pairwise differences as

$$\pi = 2 \mu L t \approx 2 \cdot 10^{-11} \cdot 10^9 \cdot 10^4 = 200$$

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  • $\begingroup$ My hypothesis was it is a significantly better for the branch to be polynated from another branch than from the same branch, This number seems really huge if 2 is a significant genetic difference in Tajima's D, doesn't it? Question is, wheter this could play a role in the evolution of the branch structure. $\endgroup$ – Probably May 31 '16 at 6:39
  • $\begingroup$ This number should be verified with better estimates (typically the mitotic mutation rate might be way too high) should probably be compared to the number of pairwise differences in the population. The number of pairwise differences between two randomly sampled indivdiuals in the population is $2 N \mu$. $\endgroup$ – Remi.b May 31 '16 at 7:54
  • $\begingroup$ I am not sure why you are talking about Tajma's D. If I remember correctly (am too lazy to make the calculations now), Tajima's D compute a standardized comparison between the number of pairwise differences number of segregating sites in a sample of size $k$ divided by $\sum_i^{k-1} \frac{1}{i}$ as both should equate under the infinite site model (coalescent theory). $\endgroup$ – Remi.b May 31 '16 at 7:56
  • $\begingroup$ Oh, sorry. But is it possible that this effect could play a role for example in the evolution of scape stem branching $\endgroup$ – Probably May 31 '16 at 13:01
  • $\begingroup$ Intuitively, I would think it is negligible but I am not sure. $\endgroup$ – Remi.b May 31 '16 at 13:57
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Having separate male and female cones reduces self-fertilization just by the physical separation (the greater distance for pollen to travel gives pollen from a different tree a larger chance of successful fertilization). A much larger effect is from the timing of flowering, since on an individual tree the female cones will be receptive at a time when the male cones on that tree are not releasing pollen.

Another reason for the different orientation of the cones is probably gravity. The male cone, hanging downwards, can have its pollen fall out of the cone into the air where wind can transport it. The female cone, pointing upwards, can capture settling pollen into its upward facing structures.

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