How Busy Are Bees - modelling the pollination of clover

Transcription

1 How Busy Are Bees - modelling the pollination of lover Athole Marshall, Terry Mihaelson-Yeates and Ingrid Williams* Geneti markers 19 Where does the pollen effetive in fertilisation ome from? 2 How far do bees arry pollen? 21 Pollen arrying apaity of bees 21 Value in risk assessment 21

2 MODELLING THE POLLINATION OF CLOVER Athole Marshall, Terry Mihaelson-Yeates and Ingrid Williams* Bees are essential pollinators for many speies of rop and wild plants that need ross-pollination for seed prodution (Figure 3.1). These speies rely on bees to transfer pollen from the anthers of flowers of one plant to the stigmas of flowers of another. The effetiveness of bees in delivering pollen from one flower to another depends on the behaviour they adopt in foraging for netar. In the past, the study of the onsequenes of their foraging behaviour was limited by the diffiulty of identifying the soure of the pollen that effeted fertilisation and seed prodution, sine pollen bears no obvious indiation of parentage. However, the Legume Breeding Group at IGER has now developed a range of novel geneti markers whih help the investigator to define Figure 3.1. Honey bee (Apis mellifera) on a white lover (Trifolium repens L.) flower. Figure 3.2 Red leaf marker of white lover 18

3 preisely whih speies the pollen omes from. In a ollaborative researh programme between plant breeders at IGER and pollination eologists at the Institute of Arable Crops Researh (IACR, Rothamsted), we are using these markers in a unique model system to measure pollen and gene flow mediated by bees. These new tehniques allow us to examine interations between plant and pollinator at a range of sales from single flowers to plant ommunities over a wide area. Geneti markers The geneti markers are all within the beepollinated, self-inompatible speies, white lover. They inlude a novel red leaf marker (Figure 3.2) whih is dominant over the normal green leaf type so that the progeny from rosses between these two lines have red leaves and an be visually identified. They also inlude five isoenzyme markers at the phosphogluose isomerase (PGI) lous of white lover, seleted within the ultivar S1 (Figure 3.3). These isoenzyme markers have the great advantage over many other markers as their seletion is not a x b a b d e a x d Figure 3.4 Experiments were onduted in pollination ages in whih plants with defined geneti markers are plaed, along with hives ontaining the different bee speies. a x a x e Figure 3.3 Simplified isoenzyme banding patterns of the five white lover PGI seletion lines, whih are used to examine pollen transfer mediated by bees. Five seletion lines of lover (green panel, labelled a - e) have a unique pattern (barode) when leaf proteins are separated on a starh gel in an eletri field and stained for ativity of the enzyme PGI (phosphogluose isomerase). When a known lover line (in this ase, line a) reeives pollen from any of the other (but unknown) lines, plants from the resulting seeds will display a more omplex but also unique barode pattern (examples are shown in the pink panels), whih allows the unknown pollen donor to be identified. The vertial arrows indiate the diretion and relative movement of the proteins in eah trak, with the position of the band shown for omparison in eah ase. This type of experiment allows the investigator to determine how foraging behaviour influenes seed prodution and paternity. assoiated with any other reprodutive harateristis whih ould influene bee behaviour. The bees annot disriminate between them. Crosses between the isoenzyme seletions an be identified in the resulting seeds using a simple laboratory tehnique whih produes a pattern similar to a supermarket barode (Figure 3.3). In our model system, we have enlosed different speies of bees on large plots of these genetially marked plants (Figure 3.4), arranged in different experimental designs, to investigate how their foraging behaviour influenes seed prodution and paternity. 19

4 Where does the pollen effetive in fertilisation ome from? To study this, olonies of bees were enlosed with a mixed plant population having equal proportions of plants of the five isoenzyme seletions. At the plant population level, the bees were thoroughly mixing the different pollens, hene they were helping to maintain geneti diversity. However, at the level of the infloresene and the individual pod, suessful fertilization was dominated by one pollen type, although usually several of the other types had also been suessful (Figure 3.5). This suggests that most of the pollen effetive in fertilisation omes from the last plant visited by the bee. % of eah paternal donor % of eah paternal donor a) Honey bee Total paternal donors within pods b) Bumble bee Total paternal donors within pods Figure 3.5 Whih plants does pollen ome from? The proportion of eah paternal isoenzyme donor type (different oloured bars) in pods ontaining one to five donors following pollination by (a) Honey bee (b) Bumble bee. % of progeny derived from eah pollen type Cross-fertilisation (from outside) Self-fertilisation (from own pollen) Honey bee Bumble bee Pollinator speies Figure 3.6 Are some speies of bees more effetive than others? Perentage of progeny derived from ross-fertilisation and selffertilisation in white lover plants pollinated by honey bees and bumble bees. Are some bee speies more effetive pollinators than others? Isoenzyme markers within both selfinompatible and self-fertile lines of white lover were used to demonstrate that honey bees transferred more ompatible pollen to flowers than bumble bees, resulting in more seeds per pod. Honey bees visit fewer flowers on eah visit to an infloresene than bumble bees and so aumulate relatively less ineffetive self-pollen onto their bodies (Figure 3.6). As they move from flower to flower within the infloresene, honey bees are therefore more likely than bumble bees to deposit ompatible ross-pollen rather than inompatible self-pollen onto the stigmas. How far do bees arry pollen? The distanes over whih bees arry pollen to other plants or other plots is an important onsideration for the avoidane of 2

5 % effetiveness of pollination Honey bee Bumble bee Distane from pollen soure (metres) Figure 3.7 How far do bees arry pollen? Pollen flow (mediated by different bee speies) was assessed by the effetiveness of pollination between bloks of isoenzyme marker plants plaed at different distanes from eah other. ontamination of seed stoks. We assessed pollen flow between two adjaent bloks of plants, whih have different isoenzyme markers, grown at different distanes from eah other. Pollen transfer was high over short distanes (less than 1 metre) but delined to low levels when plots were more than 2 metres apart (Figure 3.7), irrespetive of whether the pollen was arried by honey bees or bumble bees. sporadially up to the 9th visit. Speies therefore differ in their pollen arrying apaities and their ontribution to pollen dispersal. Value in risk assessment The availability of these geneti markers and our ombined expertise has enabled us to develop this model system for detailed studies into how bees move pollen and genes within and between plant populations. Our studies are produing exiting new information that will help quantify pollen transfer and dispersal in inset-pollinated rop and wild plant populations. Suh information is urrently of partiular importane in relation to risk assessment of the environmental onsequenes of the release of transgeni inset-pollinated rop plants. Contat: athole.marshall@bbsr.a.uk ingrid.williams@bbsr.a.uk *Ingrid Williams (IACR - Rothamsted) is an Honorary Researh Assoiate of IGER Pollen arrying apaity of bees We studied arrying apaity by allowing different speies of bees to visit a pollen donor, in this ase with the red leaf marker, and then following the deposition of that pollen onto the stigmas of flowers of normal green leaved plants next visited by the bee. Most honey bees deposited the pollen in the next 15-2 infloresenes they visited and then only sporadially up to the 5th infloresene. Bumble bees deposited the pollen more onsistently than honey bees to the next 25 infloresenes they visited and then more 21