Entomology: June 7, 2006

Today's topic: The Reproductive Biology of Fire Ant Societies

Lecture primarily based on the following paper:

Tschinkel, Walter R., 1998. "The Reproductive Biology of Fire Ants." Bioscience 48 (8): 593-605.

I.Introduction

  1. Origin and distribution of fire ants in the U.S.
  2. Introduced accidentally into Mobile, AL, in imported nursery plants

a)Two species in the genus Solenopsis

  1. Spread throughout the southeastern U.S.
  2. Effects?
  1. Usefulness as a study organism
  2. Easy to collect and maintain
  3. Easily-manipulated experimental animal
  4. Many aspects of its biology are not highly specialized, and thus applicable to a wide variety of ant species (not true for its reproductive behavior!).

II.Basics of Solenopsis invicta societies

  1. Eusocial; live in colonies
  2. Colonies consist of a subterranean region and a mound region, both with extensive chambers
  3. Considered to be an r-selected or “weedy” species
  4. Initial colonizers of disturbed areas

a)Good dispersal mechanisms (i.e. the winged alates)

b)High reproductive rates

  1. Colonies vary in terms of numbers of queens (key focus of paper).
  2. Monogyne: single queen
  3. Polygyne: multiple queens
  4. Relationships between monygyne and polygyne populations:

a)Come from the same genetic stock, single populations can be a mix of monogyne and polygyne societies.

b)The monogyne seems to be the original type, with the polygyne system developing for unknown reasons

III.Details of monogyne and polygyne societies (see table)

IV.What causes there to be both the monogyne vs. the polygyne colony forms? Not known, but two key hypotheses:

  1. Founder effect: Only a small subpopulation came to the U.S. This may have decreased genetic diversity to the point where discrimination of one’s own relatives (via odor) can break down, thus allowing for the polygyne condition.
  2. Does this hypothesis assume there is an adaptive value for the two types?
  3. NOTE: polygyne colonies also exist in Argentina (where ants originated), but there, the queens are related
  4. High population densities of fire ants in the U.S. have reduced the chances of founding new colonies.

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Differences in the reproductive biology of monogyne vs. polygyne colonies

Monogyne: single queen prevails (colonies may be co-founded) / Polygyne: multiple queens
A. Establish-ment of colonies / Method 1: Colony founding by solo queens
  • Alates released from colonies; nuptial flight—female mates once
  • Once mated, an individual queen may establish a colony by herself
  • Site selection, dealation, digging of nest
  • Initially produces a brood of minims (50-90) from her own body reserves.
  • Produced quickly, smaller than subsequent broods
Method 2: Cooperative colony founding ultimately 1 queen
  • If there is a high density of new queens in an area, queens may join to produce their initial broods.
  • A single queen wins out in the end (thus, still “monogyne” despite cooperative founding)
  • Why found nests cooperatively if there is only one winner? [Fig. 3]
  • Greater number of minims higher chance of winning brood raids (see next section)
  • Four queens produce the most minims
  • Why is there no advantage to >4 queens?.
  • The nest itself is more likely to survive when there are more founding queens (levels off at 7 queens)
Brood raiding phase (occurs regardless of solo or co-op founding)
  • First generation produced by the founding queen(s) raid each others nests for brood (more raiding when nests are closer...)
  • Stolen workers abandon their own queen and work on behalf of the new colony, even though they are unrelated!
  • The losing queen may enter the winning nest. If more of her own workers are there, she can actually displace the original queen.
  • “Raids may last weeks and include several dozen nests.”
  • Ultimately, the biggest nest wins.
  • If >1 queen left, they will fight each other and attempt to take over the brood pile
  • Eventually, all but one will be executed by workers.
  • The winner is not necessarily the one who has the most related workers
  • The heaviest queen is likely to be the winner
“Dependent” founding: An individual queen may enter an already existent nest and establish herself there. Will work only if the colony is “orphaned”
  • Alates doing this are different; do not have sufficient metabolic reserves for independent founding
  • Tends to occur in saturated habitats where all available colony sites are taken.
  • Workers already there will help rear her offspring even though they are unrelated. Considered “social parasitism”
  • NOTE: If alates are present when the queen dies, they may dealate and produce male alates from unfertilized eggs. In this case, they will not accept the new queen.
/ Colonies founded by fission
  • New colonies are usually produced by fission
  • Workers and some queens “emigrate through underground tunnels to form a new mound some distance away”
  • Independent founding is unlikely due to
  • Low percentage of alates, since alate production/survival is suppressed by pheromones from multiple queens
  • Smaller alates with body reserves that are insufficient for colony founding
  • Many diploid, and thus sterile, males (due to inbreeding...): these males are homozygous at the sex-determining locus.
  • Only males from unfertilized eggs (haploid males) are fertile. Thus, fewer female alates able to mate with polygyne males
  • Female alates mate primarily with monogyne males

Adoption of queen(s) into colony /
  • Will occur only in colonies without a queen or viable alates. Otherwise she is killed.
/
  • Adoption of both mated and non-mated queens into an established colony is the norm.
  • High fecundity queens are killed (see last section).

Social composition /
  • One queen plus workers at first
  • Once large enough (10% of its maximum size) alates will also be produced
  • Proportion of alates is variable; more produced Jan-July [Fig. 8]
  • Approximately 30% of colony’s energy goes into alates once the colony begins to produce them
/
  • Several queens plus workers at first
  • Much lower proportion of alates than in monogyne colony

Treatment of intruders /
  • Discriminate and kill
/
  • Don’t discriminate, possibly because inbreeding has reduced genetic variability to the point where discrimination isn’t possible.

Relationship of colony members /
  • Extensive genetic analysis of mature colonies has shown that the queen mates only once; thus workers are, on average, share 75% of genes
  • 5-7 million sperm last her lifetime
  • On average, only 3 sperm released in each fertilization event (very efficient!)
  • NOTE: At first, workers may be unrelated due to brood raiding scenario.
/
  • Degree of relatedness depends upon number of queens.
  • More queens decreases relatedness among workers.
  • Budding of colonies helps maintain relatedness amongst workers and the offspring they raise,but still chance that workers are raising unrelated offspring.

Regulation of reproductive ability within colony /
  • The single queen’s pheromones regulate the number/proportion of alates that will be produced,and also keep those alates from reproducing within the home colony. She also allocates male (reproductive alate) vs. female production
  • The more fecund the queen, the more suppressive pheromone she produces.
  • Queen is capable of influencing 250,000 workers with her pheromones
/
  • The females appear to mutually suppress each other, with some females winning out over others
  • Overly fecund females are killed
  • These queens have a “aa” genotype at the phosphoglu-comutase (pgm-3) locus
  • Not detected until they reproduce

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