Do sword lilies use enough protection? Yes, post-pollination reproductive barriers work!

Sword lilies are stunning, cormous perennials belonging to the genus Gladiolus in the iris family (Iridaceae). The speciation of this genus originated from South Africa and moved northward to the Mediterranean Basin leading to different flower morphologies, pollinators and ploidies. 

One of the most common species of the genus is Gladiolus communis which is known to have cytotypes (i.e. individual plants of the same species has different chromosome numbers). Zones of genetic mixtures have been described as ‘natural laboratories for evolutionary studies’ as back as 1988. One theory for mixed polyploid populations is that one population can outcompete the other.

The most recent project by Castro and colleagues from the University of Coimbra and University of Guelph (Canada) investigated if there are reproduction barriers (e.g. different pollinators, flower morphology or timing of flowers) in tetraploid-octoploid contact zones in the Iberian Peninsula. The scientists found weak pre-pollination but strong post-pollinator barriers mainly due to selfing and gametic selection, revealing that mixed polyploid populations can co-exist in a stable manner. 

Dr Mariana Castro at the University of Coimbra (Portugal) wrote her PhD project on “Evolutionary ecology of polyploids: understanding species coexistence at the contact zones” in 2018. Her research project focused on Sheep’s-bit (Jasione montana) and Gladiolus communis and the role of polyploidisation and contact zones of higher-level polyploids. 

In the Gladiolus communis tetraploid-octoploid contact zone of central Portugal, researchers assessed flowering phenology, flower morphology and pollinator behaviour. For recording pollinator behaviour, Castro and colleagues monitored three arrays of 10 flowers from the six populations between 9 am to 4 pm. In order to experimentally test reproductive barriers, bulbs from three tetraploid and three octoploid populations were used in a common garden study at the Botanical Garden of the University of Coimbra.

A total of 102 plants were used for numerous hand-pollination experiments to test the role of pollen load of self-pollination, outcross within cytotypes and outcross between cytotypes between single and mixed ploidy plants. Flowering phenology and morphology were also recorded for the common garden experiments and the scientists calculated Reproductive Isolation (RI) indexes based on each barrier for tetraploids and octoploids separately. Finally, 250 plants consisting of 1:1 proportions of tetraploid and octoploid plants were left to open pollination to assess offspring production (i.e. tetraploids, octoploids and hexaploids).

Castro and colleagues found that the flowering phenology, flower morphology and main pollinator species’ behaviour patterns was almost the same for the tetraploid and octoploid Gladiolus communis. In the controlled pollination experiments, the reproductive success was higher for octoploids which produced a few hexaploid seeds. Pollinations within the same cytotypes and among mixed ploidy treatments produced mostly offspring with the same ploidy as the parents or maternal plant. Overall, very high Reproductive Isolation values were calculated for all hand-pollination and open pollination experiments. 

The researchers explained, “because the composition of the pollen load determines both cytotype fitness and offspring ploidy, and contact zones are characterized by different mixed-ploidy spatial arrangements, the interactions between cytotypes are expected to be complex in natural contact zones”. They conclude, “this study reveals the key role of gametic selection in cytotype siring success and highlights the importance of comprehensive estimates across multiple reproductive barriers to understanding cytotype interactions at contact zones”.

Whilst the different cytotypes cannot be distinguished by the naked eye, it is interesting that octoploids had a slightly higher reproductive success. In South Africa, the biodiversity hotspot for genus Gladiolus, diploid sword lilies are the most common whilst tetraploids and then octoploids are more common moving upwards to the Mediterranean Basin. 

To learn more about polyploid plants, join the Polyploidy Webinar series organised by Dr Mike Barker from the University of Arizona.