The
From laying workers to social
parasites.
Laying workers of the
This special issue of Apidologie addresses the biological background on
the "Capensis calamity" in a series of
review papers, original articles and scientific notes. The basic principles for
identifying A. m. capensis (Hepburn and Radloff, Radloff et al.), understanding the pheromonal
basis of pseudoqueen establishment (Wossler), and the behavioural
basis for social parasitic workers (Neumann and Hepburn)
are dealt with in depth. The physiological mechanisms causing reproductive
dominance already during larval development are presented by Calis et al. The contributions of Pirk
et al., Reece and Martin et al. deal with the problem of how parasitic workers
establish themselves in host colonies and how they circumvent the colonial defence systems. Moritz develops a model on the spread of
parasitic workers in natural populations and in man kept populations on
apiaries.
The Cape Bee Problem
The Cape bee problem was discussed under the Apimondia Bee Biology Standing Commission. Traditionally, two races of honey bees have co-existed in South Africa, a northern one (Apis mellifera scutellata) and a southern one (Apis mellifera capensis), separated by a fairly wide geographic boundary consisting of large tracts of arid climate, extensions of the Khalahari and Karoo deserts. Both subspecies are good pollinators and honey producers in their own regions. Capensis, however, has a trait scientists call thelytoky. This means that capensis worker bees can lay eggs that also develop into workers, in effect cloning themselves. These so-called pseudoqueens may be an adaptation to the winds that blow near the Cape of Good Hope, which severely affect the mating ability of virgin cape bee queens. By contrast, eggs of laying workers in honey bees found everywhere else in the world more often than not result in drones, known as arrhenotoky. It is important to realize that a small amount of thelytoky is probably present in most races or ecotypes of honey bees.
In the early 1990s, capensis bees were moved as part of migratory beekeeping into scutellata areas. The result was that worker cape bees began to enter scutellata colonies. This precipitated in quick order, queen supercedure by scutellata colonies, followed by a decline in population and colony demise. This phenomena is called "social parasitism," and has been responsible for tens of thousands of colony losses in scutellata country. The social organization of scutellata colonies appears to be pheromonally disrupted by capensis workers; they simply self-destruct. There is no immediate answer to the problem it seems, although research reported at Apimondia is continuing both at universities and through South Africa’s Plant Protection Research Institute http://www.arc.agric.za/institutes/ppri/pprimain.htm.
The capensis problem brings identification of honey bees to the fore. H.E. Hepburn and S.E. Radloff of the Apiculture Group, Rhodes University, Grahamstown, South Africa reported that standard taxonomic convention in honey bee classification, the trinomial Apis mellifera (capensis, scutellata, etc.) cannot adequately accommodate the groups they have found in the country. "Incongruities between morphometric, biological and DNA groups preclude clearly defined boundaries for southern African bees. A compromise of all characters suggests the following groups: (1) thelytokous A.m.capensis; (2) thelytokous hybrids; (3) thelytokous A.m.scutellata; (4) arrhenotokous A.m.scutellata; and 5) arrhentokous mountain bees. "The only way that ‘A.m.capensis’ can be meaningful is to precisely qualify its point of origin in the Cape Provinces." The use of the term "hybrid" is unfortunate, according to Dr. Hepburn, who said in his presentation that it was more informative to call such bees "intermediate" instead.
At first blush, the South Africa situation may not hold much meaning for honey bees elsewhere, but the identification issue can be applied to the present "Africanized" bee situation in the Americas. The so-called "hybrid zone" in Argentina, which includes both European or African subspecies or mixtures may be the same kind of phenomenon as found between capensis and scutellata. With reference to the "Africanized" bee in the Americas, Dr. Hepburn said in his judgement they were simply nasty, little bees from Pretoria in most of their tropical range, when queried for his opinion at the meeting. How the Africanized honey bee will play out in temperate North America is still in question and is one of the quintessential beekeeping conundrums that must be faced in the future. It is of more than passing interest that Africanized honey bees in Arizona have in fact been determined to have a higher degree of thelytoky than their European sisters . This suggests a biological reason for requeening failures sometimes reported by beekeepers trying to introduce European stock into tropical America.
The behavioral basis of social parasitism was discussed by Peter Neumann of Martin-Luther-Universität Halle-Wittenberg, Institut für Zoologie. Beyond thelytoky, the social parasitism evoked by capensis pseudoqueens requres a high ovarial and pheromonal development, a long reproductive period (3-5 months) and the capacity to lay up to 200 eggs per day. The development of pseudoqueens is particularly well expressed in colonies of other honey bee subspecies (i.e. scutellata), so that they are able to develop retinue behavior in host workers and can suppress the rearing of replacement queens. This social parasitism is expressed as so-called "dwindling colony" syndrome. The resulting "capensis" calamity for South African beekeepers suggests that these invasive parasitic workers are highly virulent.
A. m. capensis workers require several behavioral traits related to both transmission and virulence according to Dr. Neumann.. Transmission means that these pseudoqueens must actively move to get into nearby colonies, perhaps by drifting. However, they also are able to find colonies up to a kilometer (.6 miles) away. Pseudoqueens must somehow avoid guard bees. In some cases, they may join swarms. Long range transmission is important as the wild A. m. scutellata host population appears to be highly infested. The author says the relative proportions of the different pathways actually leading to new infestations remains unclear.
Virulence is also an important part of the equation Dr. Neumann says. Pseudoqueens appear to avoid the queen in colonies they invade and establish dominance by getting workers to preferentially feed them while developing their pheromonal communication abilities. Whether workers show specific appeasement tactics or simply rely on a fast-track pheromonal development is not known. In addition, eggs laid by pseudoqueens must also avoid being "policed," that is eaten, by other workers, the fate of most worker-laid eggs in colonies with a high degree of arrhenotoky. To do this, they apparently lay eggs in out-of-the-way locations and also like a regular queen, one to a cell, instead of many to a cell as do most laying workers.
Other research reported in South Africa shows that larvae resulting from capensis eggs are also preferentially fed by host scutellata workers. Eventually, high numbers of A. m. capensis workers are reared by A. m. scutellata host colonies during later stages of infestation. This offspring can infest new host colonies via the individual or the colonial pathway, thereby completing the social parasitic life cycle of laying A. m. capensis workers. Although some of the behavioral traits of laying A. m. capensis workers have been described in detail, the researchers say that the basic frame work for the social parasitic life cycle appears to be still poorly understood.
Annelize Lubbe of Agricultural Research Council, Plant Protection Research Institute reported on efforts to identify pseudoqueens. Color, spermatheca size, ovarial development all point to black workers as capensis with a 98 percent probability. Unfortunately no single factor can be used to make the determination. All the black bees tested which had a large spermatheca and high ovariole counts, were shown to be genetically identical, confirming that when workers of the capensis subspecies reproduce parthenogenetically for several generations, autoselection occurs directed towards a homozygous genome - a blacker bee with a bigger spermatheca. Other studies reported at the meeting confirm this size differential when looking at other characteristics such as wing measurements. This state of affairs is actually a hopeful sign, however, according to P. Kruger who said that since the capensis pseudoclone does not mix its genes readily with scutellata, it is still possible to find populations of the latter in areas remote from commercial beekeeping.
A take away message from those studying the capensis problem is that it arose from purposeful movement of colonies through migratory beekeeping and this practice perpetuates it. One way to control the phenomenon, therefore, is simply to stop transhumance in honey bees, a virtual impossibility in the modern agricultural setting. Given this situation, questions remain concerning how the capensis situation will affect beekeeping in other parts of Africa. By extension, the rest of the world may also be at risk as well. Theoretically, it would take only one capensis laying worker introduced into any local honey bee population found on this globe to change it forever.
The African(ized) Queen: New Twist Found To Hive Drama
Africanized honey bees have an unexpected advantage in the battle to keep
beekeepers from replacing highly defensive Africanized queens with gentle,
easily managed European ones.Within only one week
after their queen dies or is removed by beekeepers, Africanized worker
bees--which are female--are capable of activating their ovaries to produce viable
female eggs for re-queening the hive. That's
according to preliminary findings by Gloria DeGrandi-Hoffman
of the ARS Carl Hayden Bee Research Center, Tucson, Ariz., and Stanley S.
Schneider of the University of North Carolina at Charlotte. European worker
bees' ovaries can't start producing eggs until the queen has been missing for
at least three weeks. And, egg-laying worker bees that are queenless
typically produce male offspring. In contrast, the Africanized workers' faster,
one-week response to queenlessness, and ability to
produce a queen from their own female eggs, could explain why many beekeepers'
efforts to re-queen an Africanized hive with a docile European queen haven't
succeeded. Queens introduced into colonies that have egg-laying workers will be
attacked and killed. Scientists already knew that some kinds of African honey
bees, such as the Cape bee of South Africa, can lay viable female eggs within
one week of becoming queenless, and nurture them to
become their queen. But the ARS and University researchers are apparently the
first to observe this phenomenon in Africanized worker bees in the northern
hemisphere. Migrating from Brazil, Africanized bees are today found in Arizona,
California, Texas, New Mexico and Nevada. The scientists are developing new
tactics to foil the Africanized workers' ability to make their own Africanized
queen. DeGrandi-Hoffman reported the findings at the
Second International Conference on Africanized Honey Bees and Bee Mites, held
recently in Tucson. ARS, the U.S. Department of Agriculture's chief research
wing, was co-sponsor.
G. DeGrandi-Hoffman,
E. H. Erickson Jr., D. Lusby, and E. Lusby
Carl Hayden Bee Research and Biological Control Center - Tucson - Arizona - USA
Introduction
Thelytoky is a type of parthenogenetic
reproduction where unfertilized eggs develop into females (Suomalainen
1950). Thelytoky is common in the Cape honeybee (Apis mellifera capensis Escholtz), but it
occurs with considerably lower frequency in European honey bees (Apis mellifera L.) (Onions
1912; Jack 1917; Anderson 1963; Ruttner 1976). In
colonies with queens most worker ovaries are suppressed by the pheromone
9-oxo-decenoic acid and other substances produced by the queen (Butler and Fairey 1963), or by the presence of unsealed brood (Kropacova and Haslbachova 1971).
However, ovaries can develop and workers can lay eggs after the queen and brood
are gone (Perepelova 1929; DeGroot
and Voogd 1954; Butler 1957; Butler and Fairey 1963; Jay 1970; Kropacova
and Hasibachova 1970, 1971 ). European workers
generally lay unfertilized haploid eggs that develop into males (drones). In
rare instances, virgin queens and laying workers produce diploid eggs that
develop into females (Mackensen 1943).
Given the high frequency of thelytoky in Cape bees,
the relatively rare occurrence in domestic stocks of European bees is
unexpected, since populations capable of thelytoky
have an advantage over those in which laying worker eggs develop exclusively
into drones (Ruttner 1977). Without thelytoky, the survival of a colony rests completely on the
successful mating of a single queen which must leave the hive to mate. If this
queen does not encounter drones or does not return to the hive, a replacement
cannot be produced because female larvae of a suitable age for queen rearing no
longer exist, and because the first queen to emerge usually destroys the other
queen cells in the colony. However, if brood from laying workers could be
raised into queens the colony would have a facultative survival mechanism in
case the virgin queen is lost. Thelytoky should occur
with greater frequency in populations exposed to conditions that reduce the
chances of a queen either taking or returning from a mating flight (Moritz
1984).
A strain of honey bees (hereafter referred to as LUS) has been established from
a breeding program in which virgin queens were introduced into broodless colonies (i.e., eggs and larvae did not exist in
the colony) from November to March in southern Arizona. The purpose of the
breeding program was to select for bees that would rear queens and drones at
that time of year. Inclement weather and limited numbers of drones can occur
during Arizona winters and prevent queens from successfully mating. Thus,
introducing virgin queens at this time of year exerts pressure that could cause
the frequency of thelytoky in the population to
increase. The purpose of this study was to test for the existence of thelytoky in LUS and determine the frequency of this trait.
In addition, observations of worker bees in queenless
LUS colonies were made to compare their behavior with that reported to occur in
Cape bees.
Methods and Materials
Eighteen queenless four or five frame nucleus
colonies of LUS were established using two frames of brood (ranging in age from
eggs to pupae) from queenright LUS colonies and two
to three frames of honey and pollen. The adult bees covering these frames were
included. Different LUS colonies were used to establish each nucleus colony. As
controls, three queenless nucleus colonies of a panmictic array of commercial bee lines maintained as a
closed population (CP) (Page and Laidlaw 1982;
Severson et al. 1986) and six colonies of honey bees carrying the Cordovan (cd) mutant color marker (Laidlaw
and Page 1984) were established using the procedure described above. Entrances
of the queenless colonies were covered with screen
mesh for 24-48 hours after being established to prevent bees from drifting back
to their parent colonies. All queenless colonies were
examined three to four times weekly while brood from the previous queen was
present so that queen cells from the brood could be destroyed. After all the
previous queens' brood had emerged, the colonies were examined twice weekly to
determine when workers began laying eggs. When eggs from laying workers first
appeared in the colonies (i.e., when one or more eggs were seen), 10-20 workers
were sampled and dissected to determine the percentage with developed ovaries.
The first appearance of eggs was chosen as a means to standardize the time when
workers would be sampled, since the percentage of workers with developed
ovaries can change over time in queenless colonies
(Anderson 1963). Ovaries were considered to be developed if developing eggs
were visible in the ovarioles. The brood from laying
workers present in either worker or drone cells was sexed while in the pupal stage by removing the cell's cap, and determining
gender by the morphology of the head capsule. The presence of queen cells with
larvae being actively tended by workers was noted along with whether the cells
had a queen emerge or were destroyed by the workers.
Observations of bees on the frames were made during colony inspections. We
avoided the use of smoke during these inspections whenever possible to minimize
disruption to the workers on the frames. Sometimes during an inspection bees
were seen biting each other, or with their abdomens in the cell assuming an egg
laying position. We sampled LUS bees being bitten and dissected them to
determine if they had ovary development. Whether workers assuming the egg
laying position always deposited an egg in the cell also was determined. To
conduct more detailed observations of queenless LUS
colonies, two frame observation hives were established using one frame of brood
and another of pollen and honey along with the adult bees on the frames. The
activity of bees on the frames was observed twice daily once in the morning and
afternoon, for 30-60 mm. intervals. Observations were begun when all the brood
from the previous queen had emerged. The observation hives were not included
among the colonies used to test for thelytoky.
Results
Once all the brood emerged in queenless LUS, CP, or cd colonies, worker bees were scattered over the frames
giving the colony the distinctive appearance associated with the queenless state. Upon closer examination of bees from the
4-5 frame nucleus colonies and in the observation hives sometimes workers were
seen grasping each other with their mandibles. In a LUS observation colony,
workers were seen pulling nestmates out of the cells
in which they had inserted their abdomens. On other occasions, in the
observation hives we saw eggs being eaten by nestmates
immediately after the laying worker removed her abdomen from the cell. In the
observation hives and the nucleus colonies some bees assumed an egg laying
position in a cell, but did not lay an egg. In nucleus and observation colonies
we observed bees remaining stationary with their wings spread while nestmates bit them on the dorsal surface of the abdomen and
the thoracic area (particularly at the points where the wings articulate). This
behavior occurred in LUS, CP, and cd colonies and has
been previously described in queenless colonies by Velthuis (1970). LUS from nucleus colonies that were being
bitten by other workers were examined for ovary development; 26.7% of these
bees had developed ovaries (colonies sampled = 5, total bees examined = 15, SD=
11.4%). We attempted to sample bees being bitten in CP and cd
colonies and examine them for ovary development, but sample sizes were too
small to obtain meaningful results. Dead bees on the bottom boards of seven LUS
colonies were examined and an average of 1.5% of the dead bees per
colony had developed ovaries (bees examined = 65, SD = 1.5%). Examination of
workers selected at random from the queenless test
colonies indicated that an average of 27.1% of the LUS workers had developed
ovaries when eggs first appeared in the colony (Table 1). This was a
significantly lower percentage than either CP or cd
(60.0% and 44.0% respectively).
|
Table 1 |
||||||
|
Types of progeny
reared from the eggs of laying workers in queenless
colonies of U.S. honey bees. Tucson, Arizona. |
||||||
|
Colony |
No. of |
% |
%
|
No. of |
||
|
CP |
3 |
60.0+_24.5a |
100.0 |
00.0 |
00.0 |
0 |
|
cd |
6 |
44.0+3.3a |
100.0 |
00.0 |
20.0 |
0 |
|
LUS |
18 |
27.1+15.0b |
100.0 |
55.6 |
50.0 |
9 |
|
Means followed by the
same letter are not significantly different at the 0.05 level as determined
by Duncan's [1951] multiple range test. |
||||||
Of the 18 colonies of LUS tested for thelytoky, 55.6%
reared worker brood from the eggs of laying workers, and 50% reared queens.
Queens from the brood of laying workers emerged only in the 4-5 frame nucleus
colonies, and never in the observation hives. In the nucleus colonies,
sometimes a patch of worker brood was produced and the queen cell was
constructed within that patch (Fig 1.). A queen cell positioned among worker
brood is commonly seen in a colony that is requeening
itself in the conventional manner using brood from the previous queen. However,
some queen cells from thelytokous LUS colonies were
located at the very top of the frame. Neither CP or cd
constructed queen cups in this region. A queen produced from laying worker eggs
successfully mated and produced worker and drone brood. However, eight of the
nine queens produced from workers' brood either did not return to the hive
after a mating flight, or were critically injured during artificial
insemination.
Queenless CP colonies reared only drones, although
queen cells were constructed and eggs from laying workers were placed inside
them. The eggs did not hatch, and often were gone the next day. Similarly, cd colonies produced only drones from laying worker eggs,
although some colonies reared larvae in queen cells. These queen cells were
larger and longer than those produced by LUS or commonly seen in colonies
rearing queens from a mated queen's brood. During colony inspections the cd workers were observed crawling over the capped queen
cells just as the LUS bees did in their colonies. However, within 3-5 days in
the cd colonies the queen cells were torn down by the
workers.
Discussion
LUS were selected from commercial European honey bee stock, indicating that thelytoky may exist as part of the overall Apis mellifera gene
pool. However, reports indicate that in managed colonies thelytoky
is expressed at a very low frequency (Mackensen
1943). This may be because beekeeping practices inadvertently select against thelytoky. For example, swarming and supercedure
can be minimized through various management techniques, and thus the
possibility of a colony becoming queenless due to the
loss of a virgin queen can be reduced. If colonies lose their queens and do not
have brood to produce replacements, the queens often are replaced with new ones
by beekeepers. Hence, there is no selective pressure for thelytoky
in colonies managed in this manner. Conversely, the conditions under which the
LUS strain was derived may have inadvertently selected for thelytoky.
Virgin queens introduced into broodless colonies
during the winter may not have been accepted by the workers in some cases,
while in others the queens may not have mated or were lost on mating flights.
Some of the colonies that survived may have done so because they requeened themselves with brood from laying workers. The
winter requeening procedure was repeated annually
using queens produced from brood of colonies that survived the previous year's
winter requeening. If thelytoky
was at a low frequency in the LUS strain at the beginning of the breeding
program, the frequency possibly was increased because of continued selection
followed by the production of new queens from brood of the survivors.
Unfortunately, all but one of the queens produced from laying worker brood were
lost before they could begin egg laying. Still, queens reared from the brood of
LUS laying workers apparently have the potential to mate and produce worker and
drone brood. We stopped finding eggs in the colonies once the queen emerged and
was present in the hive. The colony whose queen successfully mated, behaved
like any other colony with a new queen. After mating the queen began laying
worker brood which was cared for by the adult workers in the colony. The
colonies that reared queens but lost them did not rear others. The colonies
subsequently dwindled and died or were robbed by workers from other colonies
thus causing the LUS workers to abandon the hive. Colonies composed of 4-5
frames of workers and brood apparently have only one chance at rearing a queen
from laying worker brood. If the queen is lost, the workers will not produce
another perhaps because the workers are too old, the colony is too weak, or
some combination of both. Whether a colony that had a larger population at the
time of queen removal would have enough bees of the appropriate age to rear
another queen from laying worker brood if they lost the first one needs to be
tested.
When queenless nucleus colonies were inspected, the
use of smoke was minimized to limit the disruption of the bees. Still, opening
a colony is
disruptive because it changes colony temperature and perhaps odor and pheromone
levels within the hive's environment. We cannot be sure of the repercussions of
opening colonies on the workers' behaviors we observed on the frames.
Observation hives enabled us to make more detailed behavioral observations of
LUS workers in queenless colonies without having to
open the colony. However, how well the results from the observation hives
mirror the behaviors of bees in the nucleus colonies is not known. Workers in
observation hives reared fewer larvae into adults compared to the nucleus
colonies, and never reared queens. Perhaps the populations were too small or
temperatures could not be properly maintained in the observation hives for
brood rearing to approach the levels seen in the nucleus colonies.
There are both similarities and differences between laying workers of Cape bees
and LUS. Cape bees can have workers with developed ovaries while brood is
present (Anderson 1963). We have not found this to occur in LUS (DeGrandi-Hoffman unpubl. data).
Internal fighting among nestmates following the
removal of a queen and a subsequent increase in the number of dead bees on the
bottom board occurs in Cape, LUS, CP, and cd bees. As
in Cape bees, most of the dead LUS bees did not have ovary development. In Cape
bees an average of 28% of the workers have developed ovaries 13 days after
queen removal, and in LUS the average is 27% when eggs from laying workers are
first seen (Anderson 1963). Significantly fewer workers in queenless
LUS colonies have developed ovaries compared to CP or cd,
suggesting that worker ovaries might be more effectively suppressed by the
presence of laying workers in LUS (Velthuis 1970).
Cape bee workers lay unfertilized diploid eggs because during ana-phase II the egg pronucleus
and the central descendent of the first polar body fuse to form a diploid
zygote nucleus (Verma and Ruttner
1983). Whether a similar cytological mechanism exists in LUS is yet to be
determined.
A honey bee colony's ability to requeen itself with
the eggs of laying workers requires not only that some workers can lay diploid
eggs, but that the workers can foster the cooperation from nestmates
needed to construct a queen cell and rear the egg into a queen. When laying
workers developed in CP or cd colonies, often queen
cells were constructed and sometimes eggs were deposited inside them. However,
the eggs were either cannibalized by other workers or left unattended and did
not hatch. Other than in LUS, the greatest cooperation among individuals to
rear a queen from laying worker eggs was in cd bees
where workers actively cared for the larvae in the cells. Queen cells were
capped in some instances, but were destroyed soon afterwards. Our study
indicates that attempts at requeening occur in non-thelytokous lines of honey bees, but apparently these bees
lack some of the physiological and behavioral attributes needed to rear a
viable queen.
Acknowledgments
The authors would like to thank H. H. Laidlaw, R. E.
Page, and A. Cohen for reviewing earlier versions of this manuscript.
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Abstract
A strain of U.S. domestic honey bees (Apis mellifera L.) with the ability to rear workers and queens
using the eggs of laying workers has been isolated. Previously, thelytoky was assumed to occur rarely in honey bees with
the exception of the South African Cape bee (A. mellifera
capensis). Our thelytokous
line, hereafter referred to as LUS, was developed from commercial stocks of
European honey bees. Comparisons of worker behavior and ovarian development
were made among queenless colonies of LUS and two arrhenotokous lines hereafter referred to as CP and cd. LUS had a significantly lower percentage of workers
with developed ovaries at the time when eggs from laying workers first appeared
in cells than either CP or cd. All three lines
constructed queen ceils and deposited laying worker eggs in them, but viable
queens emerged only from LUS. The CP line did not rear larvae in the queen
cells but in some instances the cd line did. However,
the cd bees destroyed the queen cells either prior to
or soon after capping them. Comparisons between behaviors of queenless LUS colonies and those reported to occur in queenless Cape bee colonies also are discussed.
Contact Address:
Drs. DeGrandi-Hoffman and Erickson: Carl Hayden Bee
Research and Biological Control Center, U.S.D.A. - A.R.S., 2000 East Allen Road,
Tucson, AZ 85719; Mr. and Mrs. Lusby: Rangeland Honey, 3832 Golf Links Road,
Tucson, AZ 85713
Dr. Gloria DeGrandi-Hoffman is a Research Entomologist at the U.S.D.A.
- A.R.S.
Carl Hayden Bee Research Center in Tucson, AZ. Dr. Hoffman was a 1983 graduate
of Michigan State University under the direction of Dr. Roger Hoopingarner. Her research efforts have been devoted to the
construction and validation of computer simulation models of biological
systems. Dr. Eric Erickson Jr. was a 1976 graduate of the University of Arizona
and is the Research Leader and Center Director of the Carl Hayden Bee Research
Center. Dr. Erickson's research has focused on crop pollination and honey bee
behavior and morphology. Delores and Edward Lusby are commercial beekeepers in
Tucson, AZ.
Accepted for publication on 29 January 1991