The following passage is adapted from Ricki Lewis, "Did Donkeys Arise from an Inverted Chromosome?", originally published 2018 in PLOSOne Blogs.

In the world of genome sequencing, donkeys haven’t received nearly as much attention as horses. But now a report on a new-and-improved genome sequence of Willy, a donkey (Equus asinus) jack 5 born at the Copenhagen Zoo in 1997, appears in the new issue of Science Advances, from Gabriel Renaud, of the Centre for GeoGenetics, Natural History Museum of Denmark. (A female is a jenny or jennet.) The new view provides clues to how donkeys may have branched from horses along the tree of evolution.

Horses and their relatives, past and present, are genetically peculiar in that their chromosomes are rearranged, with respect to each other. That should  prevent them from producing viable hybrids – yet they do. Donkeys have 62 chromosomes and horses have 64. A mule comes from the mating of a male donkey and a female horse, and has 63 chromosomes. Mules are known for their  intelligence, calm, stamina, and persistence. Their horse-like bodies perched on donkey-like limbs make them ideal for hauling tourists around the Grand Canyon and schlepping supplies in combat situations. The ears are large like those of the horse mom, and mules make a sound that begins as a whinny and becomes a bray.

The complementary couple, a female donkey and a male horse, produces a hinny, smaller than a mule. Hinnies are the flip side of the mule, with a donkey’s physique atop horsey limbs, and short donkey ears. They’re rarer than mules, but also have 63 chromosomes. It’s easy to mix them up.

Comparing Willy’s genome to a horse genome revealed their close evolutionary relationship. [Sentence 1]Only about 15% of horse genes aren’t also in the donkey genome, and only about 10% of a donkey’s genes don’t have counterparts in the horse. Most of the genes that they share provide basic “housekeeping” functions like dismantling proteins, repairing DNA, enabling embryonic development, and controlling cell division. So that’s why a copy of each genome can smush together to yield mules and hinnies.

A second form of information encoded in genomes, in addition to the A, C, T, G sequence, is the pattern of whether the two variants of individual genes are different (heterozygous) or the same (homozygous). Many contiguous homozygous genes form a “run of homozygosity” (ROH).

[Sentence 2] An ROH indicates a chromosome chunk, perhaps as long as millions of DNA bases, that’s the same from each of an individual’s parents, who in turn inherited it from a shared ancestor, like a grandparent that cousins share.  [Sentence 3] The longer the ROH, the more recent the shared ancestor, because it takes time for mutations to accrue that would break the sameness of the sequence.

Scrutinizing ROHs can reveal recent inbreeding and domestication, help to reconstruct possible branching patterns of evolution, and, more practically, help ancestry companies assign the DNA in spit samples to geographic areas where people’s ancestors might have come from. The new study compared ROHs for the three zebra and three ass species, confirming that Willy’s most recent ancestors were Somali wild asses.

The researchers used Chicago HiRise assembly technology to up the quality of Willy’s genome sequence. [Sentence 4] “This new assembly allowed us to identify fine chromosomal rearrangements between the horse and the donkey that likely played an active role in their divergence and, ultimately, speciation,” they write. 

The bigger pieces enabled them to zero in on DNA sequences where chromosomes contort, such as inversions (where a sequence flips) or translocations (where different chromosome types exchange parts). These events could have fueled the reproductive isolation of small populations that can expand into speciation.

If eventually sperm with one inverted chromosome fertilized eggs with the same inversion, animals would have been conceived in which both copies of the chromosome are inverted – and they’d be fertile with each other, but not with horses. Once a subpopulation with the inversion became established, further genetic changes would separate them further from the ancestral horse.

The following passage and corresponding figure are from Emilie Reas. "How the brain learns to read: development of the “word form area”", PLOS Neuro Community, 2018.

The ability to recognize, process and interpret written language is a uniquely human skill that is acquired with remarkable ease at a young age. But as anyone who has attempted to learn a new language will attest, the brain isn’t “hardwired” to understand written language. In fact, it remains somewhat of a mystery how the brain develops this specialized ability. Although researchers have identified brain regions that process written words, how this selectivity for language develops isn’t entirely clear. 

Earlier studies have shown that the ventral visual cortex supports recognition of an array of visual stimuli, including objects, faces, and places.[Sentence 1] Within this area, a subregion in the left hemisphere known as the “visual word form area” (VWFA) shows a particular selectivity for written words. However, this region is characteristically plastic. It’s been proposed that stimuli compete for representation in this malleable area, such that “winner takes all” depending on the strongest input. That is, how a site is ultimately mapped is dependent on what it’s used for in early childhood. But this idea has yet to be confirmed, and the evolution of specialized brain areas for reading in children is still poorly understood.

In their study, Dehaene-Lambertz and colleagues monitored the reading abilities and brain changes of ten six-year old children to track the emergence of word specialization during a critical development  period. Over the course of their first school-year, children were assessed every two months with reading evaluations and functional MRI while viewing words and non-word images (houses, objects, faces, bodies). As expected, reading ability improved over the year of first grade, as demonstrated by increased reading speed, word span, and phoneme knowledge, among other measures.

Even at this young age, when reading ability was newly acquired, words evoked widespread left-lateralized brain activation. This activity increased over the year of school, with the greatest boost occurring after just the first few months. Importantly, there were no similar activation increases in response to other stimuli, confirming that these adaptations were specific to reading ability, not a general effect of development or education. Immediately after school began, the brain volume specialized for reading also significantly increased.[Sentence 2] Furthermore, reading speed was associated with greater activity, particularly in the VWFA. The researchers found that activation patterns to words became more reliable with learning. In contrast, the patterns for other categories remained stable, with the exception of numbers, which may reflect specialization for symbols (words and numbers) generally, or correlation with the simultaneous development of mathematics skills.

What predisposes one brain region over another to take on this specialized role for reading words? Before school, there was no strong preference for any other category in regions that would later become word-responsive. However, brain areas that were destined to remain “non-word” regions showed more stable responses to non-word stimuli even before learning to read. Thus, perhaps the brain takes advantage of unoccupied real-estate to perform the newly acquired skill of reading.

These findings add a critical piece to the puzzle of how reading skills are acquired in the developing child brain. Though it was already known that reading recruits a specialized brain region for words, this study reveals that this occurs without changing the organization of areas already specialized for other functions. The authors propose an elegant model for the developmental brain changes underlying reading skill acquisition. In the illiterate child, there are adjacent columns or patches of cortex either tuned to a specific category, or not yet assigned a function.[Sentence 3] With literacy, the free subregions become tuned to words, while the previously specialized subregions remain stable.

The rapid emergence of the word area after just a brief learning period highlights the remarkable plasticity of the developing cortex. In individuals who become literate as adults, the same VWFA is present. [Sentence 4] However, in contrast to children, the relation between reading speed and activation in this area is weaker in adults, and a single adult case-study by the authors showed a much slower, gradual development of the VWFA over a prolonged learning period of several months. Whatever the reason, this region appears primed to rapidly adopt novel representations of symbolic words, and this priming may peak at a specific period in childhood. This finding underscores the importance of a strong education in youth. The authors surmise that “the success of education might also rely on the right timing to benefit from the highest neural plasticity. Our results might also explain why numerous academic curricula, even in ancient civilizations, propose to teach reading around seven years.”

The figure below shows different skills mapped to different sites in the brain before schooling and then with and without school. Labile sites refer to sites that are not currently mapped to a particular skill.