Keeping up appearances - COnnecting REpositoriescore.ac.uk/download/pdf/82540043.pdfKeeping up...

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Orchids are one of the mostfascinating and diverse plantgroups. With an estimated 18,500species worldwide they showremarkable floral diversity andreproduction strategies. Plantbreeders have found that it isoften easy to produce hybridsartificially between species butthis does not often occur in thewild. So one of the key questions

about these plants is howspecies that occur together andflower at similar times preventhybridisation. One strategydeveloped by members of theOphrys group of orchids hasbeen to attract just one speciesof pollinator for each orchidspecies, a phenomenon muchinvestigated and which attractedthe interest of Charles Darwin.

But in the Mediterraneanregion, many species of orchidsco-exist, flower at similar timesand are visited by the samepollinating insects. How do suchspecies survive and avoidhybridisation? This was thequestion asked by SalvatoreCozzolino, at the University ofNaples, Saverlo D’Emerico at theUniversity of Bari and AlexWidmer at the GeobotanicalInstitute in Zurich. Reporting inthe Proceedings of the RoyalSociety (published online), theteam have examined 13 speciesof orchid that grow in the sameareas and flower at similar times.Six species pairs were identifiedas having shared pollinationinsects and seven species pairswhich use different, specificpollinators.

The team studiedchromosomal features of thedifferent species to determinewhether the karyotype of thedifferent orchids might be amechanism for a post-pollinationmeans of preventing fertilisationand therefore hybridisation.

The researchers found thatthere was a higher degree ofintrachromosomal asymmetrybetween the species pairs thatshared a pool of commonpollinators compared withspecies pairs that have differentpollinators. Moreover, they foundthat the differences in karyotypewere not significantly correlatedwith genetic divergence, furthersuggesting that co-existingorchids may have evolvedkaryotypic differences as ameans of reproductive isolation.

The authors believe theabsence of post-pollinationisolation mechanisms in speciesthat have evolved to havespecific pollinators suggests thatsuch specificity is important andmay help explain the karyotypicdifferences in species that sharecommon pollinators and thusprevent easy hybridisation in thenatural environment.

Orchids show an astonishing floral diversity yet an ability to hybridise inthe hands of breeders. Nigel Williams reports on a new study thatshows how neighbours appear to avoid the hybrid trap in the wild.

Feature

Keeping up appearances

Separation: Orchids, including this early spider orchid (Ophrys sphegodes), appear tohave evolved different methods to prevent hybridisation.