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Citrus General Discussion / Re: Why is Poncirus not considered Citrus?
« on: Today at 06:13:00 PM »I think this is the relevant statement of the paper:"Poncirus, a subject of continuous controversy since it was originally proposed to be within the genus Citrus is clearly a distinct clade that is separate from Citrus based on sequence divergence and whole-genome phylogeny."
Poncirus is treated as an outgroup because Poncirus is outside the clusters of all other Citrus. This leads the authors to the conclusion to separate Poncirus. In the images as in the principal coordinate analysis, Poncirus is not even inside the diagram shown. It forms a cluster far outside the other citrus groups.
I think this is the reason why it is considered as not belonging to Citrus in the paper.
Also in the cladogram Poncirus is placed basal to all Citrus. Which means that the researchers consider Poncirus to be older than Citrus. Poncirus would not be a split-off, but a sister genus to Citrus.
Concerning P.polyandra and pubinervia it is somewhat more complex. Because, as in the paper linked by Ilya, an introgression of C. maxima could be demonstrated in pubinervia (I can't remember for polyandra at the moment). This leads to a difficulty in the classification. obviously both are Poncirus representatives, on the other hand the hybridisation with maxima is a connection to Citrus. This is also noticeable in the certain clusters. pubinervia tends to group with Citrus in some parameters. In pubinervia, this is expressed by an assignment to Citrus and by labelling the hybrid origin with an X.
Whether a marginal, long-ago introgression and since then a separate development of pubinervia and polyandra justifies labelling as a hybrid species is a complex discussion. (Btw. I don't think that's a good idea, as introgressions are often found in plants)
I think the literature backs the idea that polyandra is a mutation of poncirus trifoliata.
Where to draw the genus line is not clear. Basically everything agrees that citrus, fortunella, and poncirus are fairly distant from atalantia, the preferred nearest aurantiodae (the ones I saw didn't use citropsis, which can hybridize). It's generally agreed that fortunella is closer to citrus than poncirus is, and farther from poncirus than from citrus. But it's not necessarily a ton closer. Fortunella has recently tended to be grouped within citrus, but that waxes and wanes.
If poncirus is freely hybridizing in the wild with citrus, it would tend to support it being a distant accession within citrus.