Different ways to have sex, yet still be a weed

Baker (1955) noticed that when mates are lacking, the ability to undergo self-fertilization will greatly enhance colonization success.
Uniparental reproduction seems to be common in colonizing species, whether it’s from a continent to an oceanic island, during a biological invasion or during range expansion (Pannell et al. 2015).
Weeds by their very nature should be emblems of Baker’s Law. They have superior colonization abilities as they’re often found in places they shouldn’t be and maybe without conspecifics.
Yet, there’s very few empirical tests of Baker’s Law in weeds despite the development of Baker’s idea around weeds. (Maybe it’s not so surprising since Baker also referenced other organisms, such as mosses and ferns for which there’s not been much empirical work, but see Krueger-Hadfield et al. 2016)
How do sexual systems of species influence their weediness?

Van Etten et al. (2017) combined a database of weed (weedy/nonweedy) and introduction status (introduced/native) of plants found in the USA with a database of plant sexual systems.
Weeds should have a higher propensity for self-fertilization than non-weeds. However, should we expect differences between native and introduced weeds? Don’t weeds, regardless of their origin, have “weediness” traits that make them such efficient colonizers of roadsides or places where we’d rather they weren’t?
Using a taxonomic approach as well as an approach correcting for phylogeny, Van Etten et al. (2017) found weeds differ from nonweeds in the distribution of sexual systems.

Figure 2 from Van Etten et al. (2017): Phylogeny with sexual system (outer ring), introduction status (middle ring), and weediness status (inner ring) indicated for each species. The 17 most common families are shaded and labeled with letters.

Introduced weeds were generally hermaphrodites, increasing the likelihood of reproducing when mates are scarce. Native weeds, on the other hand, tend to be monoecious. So, there’s support for Baker’s Law, but a different sexual system to get to uniparental reproduction.
These differences are likely due to phylogeny. The majority of introduced weeds come from three families where there’s lots of hermaphrodites. In contrast, the majority of native weeds come from two families where they’re either dioecious or monoecious.

It is notable that introduced and native species appear to be quite taxonomically different, and yet, weeds of both groups are more likely to be functional hermaphrodites compared with their respective nonweeds.

This study provides further evidence that uniparental reproduction is important for colonization success, but also that weediness, whether native or introduced, is also facilitated by an ability to undergo self-fertilization.
They do note that studies consider traits singly. There is not a single “weedy” phenotype, but there does seem to be mounting evidence that uniparental reproduction is pretty darn important to colonization success.
Baker (1955Self-compatibility and establishment after “long-distance” dispersalEvolution9347349.
Krueger-Hadfield et al. (2016) Invasion of novel habitats uncouples haplo-diplontic life cycles. Molecular Ecology 25: 3801-3816.
Pannell et al.  (2015The scope of Baker’s lawNew Phytologist208656667.
Van Etten et al. (2017) Not all weeds are created equal: A database approach uncovers differences in the sexual system of native and introduced weeds. Ecology and Evolution DOI: 10.1002/ece3.2820.

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