Life

Looking pretty leads to female sexual success, not so good for males

If you’re a plant who wants to reproduce should you have more flowers? I thought the answer might simply be a matter of the cost of producing more flowers against the chances of attracting more insects for pollination. Effects of floral display size on male and female reproductive success in Mimulus ringens by Jeffrey D. Karron and Randall J. Mitchell shows that it’s a bit more complicated than that.

Mimulus ringens flower
Mimulus ringens. Photo Matt Lavin / Flickr

You’d think more flowers make sense. As Karron and Mitchell point out, bigger displays attract more pollinators, so you should get more external pollen arriving with bigger displays. The problem is with what happens when a pollinator arrives. It’s could fly off taking your pollen to another plant. This is a win for you if you want to sire more plants. But what could happen is the pollinator flits from one of your flowers to another. The more flowers you have the more likely this could happen. So you could simply be increasing your chances of getting your own pollen back.

It’s this pollination that’s interested Karron and Mitchell. Studying female success rate is a bit easier because you can study the seeds on a plant. Studying the male success rate means being able to track back where the pollen for a seed came from. The way they did this was by genotyping the plants. When the seeds were genotyped they could identify the male and female contributions to the seed.

They then planted out arrays of plants. Each array was isolated from each other by a lot of bumble-bee friendly plants, so that each array was effectively isolated by a wall of non-Mimulus pollen. The arrays themselves were planted out to a set pattern. Each plant had 2,4,8 or 16 flowers. Each plant that was tested was surrounded by eight other plots, and for each plant the neighbours were 2×2 flower plants, 2×4 flower plants, 2×8 flower plants and 2×16 flower plants. This way no plant was in an easier or tougher spot to attract pollinators. If you’re wondering how you can know a plant has 2,4,8 or 16 flowers this isn’t down to breeding. It’s down to a pair of scissors.

How plants can be laid out in an even array.
Arrangement of genets and floral display size treatments in one of the four experimental arrays. Image by Karron and Mitchell.

The next step is to gather the fruits and work out what genetic material came from where.

What Karron and Mitchell found was that success varied according to sex. The seeds mothered per plant were pretty much constant. That means that adding more flowers makes you the mother of more seeds. However the story was very different for seeds the plants fathered.

Graphs showing the success of various floral displays
Truncated graphs of Floral Display vs. Seed production by Karron and Mitchell. The full version can be downloaded from the paper.

The number of seeds fathered per flower drop as floral display increases. You can fertilise yourself, but the gains from that are less than the losses from not siring other plants elsewhere. The results comparing outcrossing with self-siring are even greater and the result per plant is that more outcross seeds were sired by plants with four flowers than with sixteen.

The result is that plants might do better if they produce flowers over a longer period more than producing flowers in one go. In the case of M. ringens it looks like the plant has a few options. The plant is can display 1-3 flowers at a time in the wild and even more when cloned. To me, I wonder if this could give it a variety of option when colonising new areas.

If you’d like to read more you can get the paper for free from the Annals of Botany.

Papers that take this research further include:
Nunziata et al. 2012. ‘Characterization of 42 polymorphic microsatellite loci in Mimulus ringens (Phrymaceae) using Illumina sequencing’ American Journal of Botany. doi: 10.3732/ajb.1200180 (Open Access)
Morales et al. 2012. ‘Sterile flowers increase pollinator attraction and promote female success in the Mediterranean herb Leopoldia comosaAnnals of Botany doi:10.1093/aob/mcs243
Harder and Prusinkiewicz 2012. ‘The interplay between inflorescence development and function as the crucible of architectural diversity’ Annals of Botany doi:10.1093/aob/mcs252

Photo Mimulus ringens by Matt Lavin. [cc]by-sa[/cc]

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