Varroa mites on honey bee pupa.
By Unknown author - Kika De La Garza Subtropical Agricultural Research Center
Weslaco, Texas, USA, Public Domain, Link
Weslaco, Texas, USA, Public Domain, Link
Creationists like to portray living things as products of a single act of creation in which every species was equipped with its characteristic form and method of reproduction from the outset. In that view, reproductive systems should be fixed features of separately created “kinds”, not historically contingent arrangements that can be remodelled by natural selection.
Biology reveals something very different. Even such fundamental characteristics as sex determination, chromosome number and the balance between sexual and asexual reproduction can evolve. Existing developmental mechanisms are modified, discarded or repurposed according to whether the resulting variations leave more descendants. Evolution has no predetermined direction, no obligation to proceed from “simple” to “complex” and no rule forbidding a lineage from reacquiring a feature resembling one possessed by its ancestors.
An international team of researchers has now discovered an especially striking example in Varroa destructor, the globally invasive parasitic mite that feeds on honeybees and transmits damaging viruses between them. Its reproductive system appears to have reversed what was once regarded as an evolutionary one-way street.
Like many mites, bees, wasps and ants, Varroa was believed to be haplodiploid. In conventional haplodiploidy, females develop from fertilised eggs and possess two sets of chromosomes, while males develop from unfertilised eggs and possess only one. Consequently, a haploid male can transmit only the single copy of each gene inherited from his mother.
That should present Varroa with a serious evolutionary handicap. A female mite enters a honeybee brood cell shortly before it is sealed and produces one son followed by several daughters. The siblings mate with one another inside the cell, producing generation after generation of extreme inbreeding. Combined with the population bottlenecks associated with invading new hosts and new geographical regions, this should rapidly erode genetic diversity and limit the parasite’s ability to adapt.
However, by tracing genetic variants through three-generation pedigrees, Nurit Eliash and colleagues discovered that Varroa is not conventionally haplodiploid after all. Its females produce sons parthenogenetically, but those sons initially inherit both copies of their mother’s genome and are effectively genetic clones of her. Although DNA is subsequently eliminated from some of the males’ body cells, their reproductive lineage retains access to both maternal copies, either of which can be transmitted to a daughter.




































