As April approaches, scientists have settled one of nature's biggest crimes, an "Easter egg counterfeiting scandal" that has been brewing for 2 million years, as people in some countries prepare to unravel their Easter eggs.
Around the world, many birds lay their eggs in the nests of other species, thus escaping the cost of parenthood. This way of living is called nest parasitism. It naturally has many advantages, but it also brings many challenges, such as how to "convince" or deceive other species into accepting an alien egg.
Many nested parasitic birds mimic the color and pattern of their host's eggs, however, some nesting parasitic birds utilize several different host species, and eggs from different species may look different. So, how can a nesting parasitic bird imitate several completely different bird eggs at the same time? How did these parasitic pretenders pass on this ability to their offspring?
These questions have puzzled scientists for more than a century. Now, a major breakthrough has been made in genetic research by an international team. The study focused on the egg imitation genetics of the parasitic weaver finch (Anomalospiza imberbis), a bird species that adopts a parasitic lifestyle and uses many species of warblers across Africa to help brood.
However, the new findings suggest that, from an evolutionary perspective, this deception may have left the parasitic weavers in a cocoon, and that today, the victims may have gained the upper hand in evolution.
Maternal inheritance
The sex-determining system we are most familiar with is XY, which we and many mammals use. In this system, males are so-called heterogeneous sexes with different X and Y sex chromosomes.
However, birds and some insects use a different system, known as the ZW sex determination system, and in contrast to the XY system, the female with heterogeneous chromosomes (ZW) is the opposite.
XY Gender Determination System and ZW Gender Determination System. (Figure/Principle)
The team collected a large amount of field data at a study site in southern Zambia and in surrounding communities. Parasitic weavers in this area can trick grass warblers from 4 species, causing devastating effects.
The team collected DNA samples from 196 parasitic weaver finches from 141 bird nests belonging to four grass warbler species and sequenced thousands of short fragments of their genomes. The results showed that the female parasitic weaver sparrow inherited from the mother the ability to imitate the appearance of the host bird's egg through the female's unique W sex chromosome.
This maternal inheritance allows parasitic weavers to avoid the risk of falsely imitating genes inherited from fathers raised by different hosts, thus allowing different lineages of female parasitic weavers to evolve specialized imitations for different host species.
At the same time, they know how to further diversify bird eggs, mimicking variations in the color and pattern of at least a few different female eggs within the host species.
This maternal inheritance also validated a hypothesis first proposed by ornithologists in 1933. At the time, ornithologists observed that the European common cuckoo could mimic the eggs of different host species in a similar way.
Parasitic weaver finches and host chicks. (Photo: University of Cambridge)
Subtle imitations and disguises have blinded many host parents into mistakenly accepting parasitic eggs without throwing them out of the nest. Young parasitic weavers usually grow faster than their hosts' own chicks, and the poor biological flesh is often quickly starved to death. This has been the key to the success of these nest-parasitic African birds.
A double-edged sword of natural selection
However, the researchers believe that this long-established maternal genetic structure may in turn plague the parasitic weaver.
In this particular arms race of co-evolution among species, natural selection creates a double-edged sword. While maternal inheritance gives parasitic weavers the ability to take advantage of multiple host species, this may slow down their ability to evolve anti-adaptive as hosts evolve new defenses.
As a counterattack against pretenders, many host species have become skilled "quality managers", who know how to carefully screen and reject eggs that are different in color and pattern.
In particular, parasitic birds face a daunting challenge, as some host species in turn have evolved eggs of a variety of color and pattern characteristics, with an astonishing diversity, which helps hosts distinguish their own eggs from "fakes" of parasitic birds. For example, the four grass warbler species in the study have evolved the ability to deposit unique "signatures" on their eggs to enhance the detection of invaders. For example, the eggs laid by the brown-winged wren warbler (Prinia subflava) have a blue, white, red or olive-green undertone with various patterns on them.
However, due to the limitations of the genetics of the parasitic weaver mother, these parasitic birds are unable to recombine the falsified features evolved from individual lineages. For example, two lineages of parasitic weaver mothers have successfully evolved blue or red bird eggs, but they don't seem to be able to create the precise blend of pigments needed for an olive-green egg.
The parasitic weaver imitates the behavior of the brown-winged warbler bird's egg. The photograph shows the eggs of the parasitic weaver (the middle circle) and the eggs of the common host brown-winged wren warbler (the outer circle), revealing the diversity of host egg colors and patterns imitated by the "fakes" of the parasitic weaver eggs. However, parasitic weavers are unable to mimic the dark olive-green eggs of some brown-winged wren warblers (top left). This may be a limitation of the genetic limitations of parasitic weavers. (Photo: University of Cambridge)
In a previous study, scientists have found that the proportion of olive green in the host's eggs has become increasingly large, indicating that this is a counterattack of the host species's accelerated evolution. As expected, the team found that these host birds employ a genetic process different from that of parasitic weavers, namely twin parental inheritance, passing on to their offspring their "anti-fraud" abilities.
Uncertain about the future?
Scientists say the parasitic weavers missed out on a powerful source of new evolutionary traits that could cost them dearly in this ongoing arms race. A glaring flaw in the way they genetically mimicked may have made the host's defenses more effective and limited the parasitic birds' ability to respond.
At worst, the host may evolve infamous bird egg traits, which may force parasitic weavers to switch to other "dumber and more naïve" host species. Alternatively, parasitic birds may become increasingly dependent on young host individuals, who have not yet fully learned to create their own characteristics and are less adept at observing and spotting different eggs.
This long evolutionary arms race seems to have a temporary advantage, but it is difficult to say what the future holds and what kind of stories natural selection will lead to.
#创作团队:
Compile: M ka
Typography/Design: Wenwen
#参考来源:
https://www.cam.ac.uk/stories/egg-forging-evolutionary-puzzle-cracked
#图片来源:
Cover image: University of Cambridge
首图：University of Cambridge