Genome sequencing shows climate barrier to spread of Africanized bees
Added diversity may be useful for breeding bees

Date:
October 21, 2020
Source:
University of California - Davis
Summary:
Since the 1950s, 'Africanized' honeybees have spread north and
south across the Americas until apparently coming to a halt
in California and northern Argentina. Now genome sequencing of
hundreds of bees from the northern and southern limits shows a
gradual decline in African ancestry across hundreds of miles,
rather than an abrupt shift.



FULL STORY ========================================================================== Since the 1950s, "Africanized" honeybees have spread north and south
across the Americas until apparently coming to a halt in California and northern Argentina. Now genome sequencing of hundreds of bees from the
northern and southern limits shows a gradual decline in African ancestry
across hundreds of miles, rather than an abrupt shift.


========================================================================== "There's a gradual transition at the same latitude in North and
South America," said Erin Calfee, graduate student in the Department
of Evolution and Ecology at the University of California, Davis, and
first author on the paper, published Oct. 19 in PLOS Genetics. "There's a natural barrier that is likely maintained by many different genetic loci."
That barrier is mostly likely climate. Bees with majority African ancestry
are unable to survive colder winters.

European colonists brought European species of honeybees (Apis mellifera)
with them to the Americas as early as the 1600s. In addition to apiaries,
these bees established in the wild alongside native bees.

In 1957, imported African honeybees of the subspecies Apis mellifera
scutellata swarmed out of experimental hives in Brazil and started to
rapidly spread, interbreeding with the resident European bees. Native
to southern and eastern Africa, scutellata bees are known for their
defensive behavior. They also carry some useful traits for beekeepers,
such as resistance to Varroa mites.

Calfee, working with Professor Graham Coop and Associate Professor
Santiago Ramirez from the Department of Evolution and Ecology and
collaborators in Argentina, sequenced the genomes of bees collected at
the northern and southern edges of the scutellata expansion.



==========================================================================
They found that the bees at the northern and southern edges of the range
have a highly variable mix of scutellata and European bee ancestry. The
higher the latitude, the less scutellata ancestry is in the mix.

"The whole genome is tracking latitude and climate," Calfee said. There
are likely many genes involved in climate sensitivity and winter
survival, she said. But Calfee also finds that in some parts of the
genome scutellata ancestry has spread far beyond these climate limits
in both North and South America, evidence that some scutellata genes
are advantageous and not tied to climate sensitivity. In contrast,
the researchers did not find any evidence for selection for European
ancestry in the spread of scutellata bees.

The findings challenge the idea of a binary difference between
"Africanized" and "European" honeybees, Ramirez said. In fact, all of
these introduced honeybees are hybrids to some degree.

Diversity a resource for breeding bees The results of the study could be
of interest for breeding bees for desirable traits, such as resistance
to pathogens. Although the researchers looked only at gene sequences and
not the resulting phenotypes (except for one, wing length), the results
do show which genetic loci are important because they are under selection
in the hybrid zones.

Scutellata bees and hybrids have a lot more genetic diversity than
European bees, and the researchers found that they lost very little of
this diversity in their expansion from Brazil to California.

"Beekeepers can potentially draw from this genetic variation to breed
for desirable traits," Ramirez said. What started out as an invasion
may become part of the solution to declining bee health.

"It makes sense but it's kind of surprising because we have 1.5 million (honeybee) colonies being brought into California every spring to
pollinate crops," Ramirez said. Those domesticated bees are then trucked
around the country, but they do not appear to have a large effect on
the spread of African ancestry into feral honeybee populations.


========================================================================== Story Source: Materials provided by
University_of_California_-_Davis. Original written by Andy Fell. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Erin Calfee, Marcelo Nicola's Agra, Mari'a Alejandra Palacio,
Santiago R.

Rami'rez, Graham Coop. Selection and hybridization shaped the rapid
spread of African honey bee ancestry in the Americas. PLOS Genetics,
2020; 16 (10): e1009038 DOI: 10.1371/journal.pgen.1009038 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201021163936.htm

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