Wednesday, March 09, 2011

Why Has this Bee Landed on this Flower?


This is a photo from Botany Photo of the Day. The flower is Crepis barbigera.

Why is the bee visiting this flower? What is the evolutionary advantage of attracting bees? You may think you know the answer but you will be surprised by the comment at the bottom of the posting on the Botany Photo of the Day website. Nothing in biology is simple.


18 comments :

  1. Wouldn't the advantage be that, even though the flower *can* self-pollinate, being pollinated from other flowers would tend to decrease the amount of inbreeding depression?

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  2. Doesn't the question then become: why does Crepis barbigera have flowers?

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  3. Most flowers are adaptive, a plant growing a flower gains an advantage in reproduction. This flower, however, is a vestigal trait because this species (and the entire genus?) of plant reproduces asexually; the flower is not adaptive in this case, unless there is some other benefit to growing flowers (they contain compounds that deter herbivores?).

    The hypothesis to test here, I think, is "There is a fitness benefit to this individual plant to growing a flower and providing bees with nectar". The evidence "this plant does not reproduce sexually" pretty convincingly disproves that hypothesis.

    I think it's still a safe bet to guess any flower is adaptive, considering most flowers on most flowering plant species are functional and provide a benefit to the individual plant in reproduction. Then one formulates an appropriate hypothesis and sets out to test it. I'm going to go ahead and pre-emptively agree with any later comment that advises formulating "is this adaptive?" type hypotheses / research questions when encountering such biological scenarios as bees-in-flowers.

    From the bee's point of view, as long as the energetic reward (pollen, nectar, etc) gained from visiting the flower is greater than the energetic expense incurred flying to and from it, it's adaptive to visit flowers such as this one. I'm assuming here that the bee can actually find some sort of reward in such a flower, maybe this bee is making a mistake?

    Is there a side to this story that relies exclusively on molecular biology that I'm not seeing?

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  4. The hypothesis to test here, I think, is "There is a fitness benefit to this individual plant to growing a flower and providing bees with nectar". The evidence "this plant does not reproduce sexually" pretty convincingly disproves that hypothesis.

    The history of strictly asexually reproducing organisms is a sad tale of relatively rapid extinction. Before this plant lost the capacity for sexual reproduction (if it truly reproduces only asexually), the flowers' capacity to attract pollenators was assuredly adaptively advantageous.

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  5. anonymous says,

    The history of strictly asexually reproducing organisms is a sad tale of relatively rapid extinction.

    Several million species of bacteria will be surprised to hear this.

    The mystery isn't that asexually reproducing species exist—that's a fact. There are even eukaryotic examples.

    The real mystery is why sexually reproducing organisms (eukaryotes) are so common. It's why the explanation for sex is the greatest problem in evolutionary biology.

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  6. @JCCS Wouldn't the advantage be that, even though the flower *can* self-pollinate, being pollinated from other flowers would tend to decrease the amount of inbreeding depression?

    If the flower reproduces clonaly then there is no pollination of any sort going on.

    I would guess that the flower used to reproduce sexually and has since given this up.

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  7. Several million species of bacteria will be surprised to hear [that strict asexual reproducers are unsuccessful].

    Those bacteria will also be relieved that horizontal gene transfer rescues them from Muller's ratchet.

    But to the issue of the flower: you asked why they are still attractive to pollinators, inviting an adaptive answer, and used their current asexual reproductive status to discount sexual selective effects. Almost all multicellular organisms, like this flower, that are currently asexual were sexual in the evolutionary recent past. This means that sexual selection cannot be so easily dismissed as an adaptive justification for why this flower has vestigial attractiveness to pollinators.

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  8. the answer is easy. the bee landed on this flower because she finds it attractive. other thing bothers me. what the heck does the plant do with the flower. probably it an atavism, but i don't know

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  9. Is that really a bee? To me, it looks more like one of those flies that try to look like bees (i.e. wanna-bees).

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  10. The comment at the bottom of the posting of the photo does not mention whether Crepis barbigera has nectar or not.

    The questing in the heading is 'why has this bee landed on this flower?' Would the answer be: How would the bee know whether there is anything to get or not?
    Does this Crepis have honey marks in UV light?
    Is this yellow flower sitting in the midst of a fiels with nectar / pollen yielding yellow flowers, or is it a field of only parthenogenetic no-food yielding Crepis?
    How often are flowers of this Crepis species visited by bees? One bee and one flower: nothing to go on.

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  11. Maybe the plant is evolving from asexual to sexual reproduction. The first stage is attracting pollinators, then it will lose it's asexual ability.

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  12. HGT is not an answer to the lack of sex in bacteria. Yes HGT happens, but it is almost certainly not the mechanism for generating genetic diversity within the population. DNA Pol IV is a better candidate for that.

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  13. Yes HGT happens, but it is almost certainly not the mechanism for generating genetic diversity within the population. DNA Pol IV is a better candidate for that.

    Pol IV (or V) use and HGT are not equivalent. Either of the SOS response or the adaptive response, best understood in E. coli as mechanisms that invoke error-prone DNA polymerases and/or down-regulate DNA mismatch repair, are essentially the equivalent of a simply adding more generations using higher fidelity polymerases. HGT is a mechanism to vault broad fitness barriers in the adaptive landscape that are otherwise can't-get-there-from-here scenarios using point mutation based mechanisms like Pol IV.

    Pol IV is tremendously inferior because most of the genetic changes made by pol IV will be either detrimental or neutral. In contrast HGT brings in wholesale new functionality to the genome, without the huge fitness costs associated with random mutagenesis.

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  14. Bayesian Bouffant, FCDFriday, March 11, 2011 2:47:00 AM

    To get to the other side.

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  15. anonymous writes:

    HGT is a mechanism to vault broad fitness barriers in the adaptive landscape that are otherwise can't-get-there-from-here scenarios using point mutation based mechanisms like Pol IV.

    Pol IV is tremendously inferior because most of the genetic changes made by pol IV will be either detrimental or neutral. In contrast HGT brings in wholesale new functionality to the genome, without the huge fitness costs associated with random mutagenesis.

    Is there good information on (1) HGT in bacteria being non-random? (2) the incidence of detrimental or neutral effects of HGT? and (3) HGT's fitness costs?

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  16. @Larry,

    The real mystery is why sexually reproducing organisms (eukaryotes) are so common. It's why the explanation for sex is the greatest problem in evolutionary biology.

    Thinking on sex is dominated by adaptive assumptions that aren't really justified, but have a stranglehold. Sex is actually rather neutral, in adaptive terms, and deeply rooted within the clade - that's why it's so common: the clade is built upon sexual ancestry, and not just mitochondria and nuclei. For sure, there are opportunities to be reaped by its abandonment - twofold, indeed - but these are akin to the opportunities available to cells to become cancerous. Asexual reversion is a kind of 'species cancer', neither inevitable nor universally fatal, but certainly not an adaptation - and therefore sex does not qualify as a present-day adaptation either, obliged to provide offsetting benefits to compensate for its assumed costs. Sex is simply the stock modus vivendi of this huge clade, free for the isogamous and locked-in for the gendered, the odd dandelion notwithstanding.

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  17. The hypothesis to test here, I think, is "There is a fitness benefit to this individual plant to growing a flower and providing bees with nectar". The evidence "this plant does not reproduce sexually" pretty convincingly disproves that hypothesis.

    I think it's still a safe bet to guess any flower is adaptive...


    Secondary asexuals get saddled with their ancestral baggage, largely frozen at the moment an asexual genome was generated from a sexual ancestor. Opportunities for evolutionary change are then severely circumscribed, because long generation times and the need for serial change in a lineage (rather than amalgamation of changes from multiple lineages) places a severe limit on rate. Then we have clonal interference, and competition with the resident sexual and its rather efficient gene-level "R&D-and-distribution" networks. Essentially, the asexual genome is one huge 'gene', and change is veeeeery slooooooow.

    I think the expectation that this flower should disappear along with sex is misplaced, because the opportunity for adaptation is strongly conditioned by genetic system. The resident sexual could shed a flower in an evolutionary eye-blink, if there was a pathway of increased fitness accessible to do so. The asexual is stuck like that: a sitting duck.

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  18. How about...the flower diverts the bee from pollinating competing species?

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