Numbers and meaning
Others quickly built on Flemming’s work, using histological stains to pick out chromosomes in numerous animals and plants. The bottom line: the number of chromosomes in animal and plant cells varies enormously between species, but is constant for all cells of a given species (except for the half-complement in the germ cells).
There is no rhyme or reason to explain how many chromosomes a species has, nor any relationship with its biological sophistication. Flemming’s beloved salamanders have 6, fruit flies 8 and humans 46 (the exact number was not agreed until the late 1950s). Pub quiz enthusiasts may wish to note that the Australian daisy and the jack jumper ant have only 2 chromosomes; that the mammal with the most (92) is Pittier’s crab-eating rat; and that top-scoring species include the Agrodiaetus butterfly (268) and the adder’s tongue fern (an astonishing 1,260).
Chromosomes also vary markedly in size and shape. In humans, the smallest is chromosome 21, about one-fifth of the length of the largest, chromosome 1. Most chromosomes are thread- or worm-like, consisting of two paired chromatids joined together at the ‘centromere’ to create two pairs of unequal arms. The chromosomes in some species are unusually shaped, such as the massive, fuzzy ‘lampbrush’ chromosomes of amphibians (first described by Flemming, in the eggs of the Mexican axolotl), and the angular hairpin-like ones of the threadworm.
At high magnification, stained chromosomes show alternating bands of heavy and light colouration. These are not random graffiti of nature. A particular dye paints each chromosome with a diagnostic banding pattern, so reliably that it can be used to map the position of normal genes and mutations. Some mutations can be seen as a section snipped out of the normal sequence, or an extra piece slipped in between two bands that are normally adjacent.
Some of the obvious questions about chromosomes were answered relatively quickly. Did they somehow assemble themselves from the granular material inside the nucleus, in readiness for the grand act of cell division? Or were they in there all the time, buried like fossils, only to be revealed when the rest of the nucleus dissolves away? The latter appeared to be the case, as the ends of the hairpin-shaped chromosomes of the threadworm could be seen as bulges in the membrane of the nucleus when it reformed after cell division.
Next question: what were chromosomes made of? Some clues emerged quickly, but were not understood for some years. In 1872, just a couple of years after Miescher’s first paper, the Estonian botanist Edmund Russow found that the chromosomes in pollen cells dissolved in alkali – without realising that he had stumbled across the phenomenon that first alerted Miescher to the oddness of the substance that he called nuclein.
Almost a decade later, Eduard Zacharias followed up one of Miescher’s experiments, by trying to dissolve chromosomes with the protein-digesting enzyme pepsin. It failed, exactly as it had left the nuclei intact in Miescher’s pus extracts. Zacharias did not make the connection himself, but Flemming did. He wrote in 1882, ‘possibly chromatin is identical with nuclein’. This was the first intimation that Miescher’s nuclear substance might be a major component of chromosomes.
The biggest puzzle – what chromosomes did – was still unsolved by the time the old century reached its end and Flemming entered his last two years. Some scientists were excited by the suggestion that the chromosomes carried the heritable characteristics; but somewhat louder were the voices of all those who ridiculed that idea.
For such an amiable man, Walther Flemming deserved better in his premature old age. During his forties, he developed a progressive neurological illness with muscle weakness and disabling attacks of pain. This put paid to his annual bid for freedom, chasing butterflies across high Alpine meadows after the end of the academic year. His physical decline could be gauged from the growth rate of his butterfly collection: with increasing difficulty, this reached a respectable 4,290 specimens and then expanded no further. After pinning out his last butterfly in the summer of 1901, Flemming fractured his femur in a fall and retired to bed for the rest of his life.
His last big paper, on the centriole, was published in 1891. After that, he continued to correspond with fellow scientists and to defend his conclusions vigorously against all comers. Being a gentleman, he put dissenters down with a polite, well-argued résumé of the evidence. After almost five harrowing years confined to bed with his broken hip, Walther Flemming died of pneumonia in early August 1906, aged sixty-two.
* Some species of orchid are fertilised by wasps or bees which copulate (badly) with insect-shaped structures on the front of the flower.
4
GARDENING LEAVE
It promised to be quite an adventure for a twenty-two-year-old: a mission combining propaganda and espionage, crossing Germany from north-eastern France into the heart of the Austrian Empire, then heading south to Vienna for the home leg. C.W. Eichling, German born and an alumnus of Vienna University, was a travelling salesman for Louis Roempler of Nancy, specialist supplier of botanical ‘novelties’. Roempler had built up a respectable following across central Europe, and their magnificent geranium ‘Madame Roempler’ (‘immense trusses of carmine rose flowers’) had recently caused a stir in faraway Philadelphia. Armed with the new catalogue, Eichling was tasked with spreading the word, ramping up sales and bringing back intelligence. At his employer’s suggestion, he had grown a beard and moustache, for extra gravitas.
The high point of the trip should have been the visit to Erfurt in central Germany. The city was nicknamed ‘Blumenstadt’ to celebrate the prosperity brought to it by Ernst Benary, the country’s most rampant plant merchant and a grand master of ‘art gardening’. Each year, the green-fingered of Europe snapped up the new ‘Album Benary’, with its beautifully coloured lithographs and accompanying text in German, French, English and Russian. Roempler was doing well, but Benary was in the top league.
On reaching the City of Flowers, Eichling went to pay his respects to the man who epitomised flower power. While they talked, Benary mentioned ‘a prominent academic’ customer who had done amazing things with fuchsias and some strange experiments with peas. It turned out that the academic customer was the abbot at a monastery in Brünn, the capital of the Austrian province of Moravia.
Brünn was already on Eichling’s itinerary. His existing customer in this ‘quaint old city’ knew and admired the ‘beloved cleric’ but was surprised that the good abbot’s ‘putterings’ in his garden had caught the attention of the great Benary. Further surprises awaited Eichling on the fine summer morning when he called at the imposing Augustine Abbey of St Thomas, which dominated the Klosterplatz in the old city of Brünn. He had built up the mental picture of ‘an old, wrinkled, spooky monk’; instead, he found himself shaking the ‘welcoming hand’ of a ‘fine-looking, spectacled priest’ in his fifties, with a smile that conveyed ‘both determination and kindness’.
Both being native German speakers, they immediately slipped into a ‘lively conversation’. This began with the abbot grilling Eichling about Roempler’s catalogue of rare plants (and displaying exceptional knowledge in the process); continued through lunch (‘home-made bread, exquisite ham and beer’); and eventually descended into renditions of what both men could remember of the students’ drinking songs from their respective days at university.
After lunch, they went out into the sunshine to walk around the abbey gardens: extensive, beautifully laid out and ‘clean as a pin’. The abbot showed particular pride in the vegetables and fruit, and explained that he had ‘reshaped’ the peas to serve the abbey ‘to better advantage’. Prompted by what Benary had said, and probably eyeing up the commercial possibilities, Eichling asked him to elaborate. All the abbot said was, ‘It is just a little trick, but there is a long story connected with it which would take too long to tell.’ He then deliberately changed the subject and steered his visitor away from the peas, carefully avoiding any further mention of either the ‘little trick’ or the ‘long story’. At the end of the afternoon, the abbot sent Eichlin
g on his way with ‘a hearty handshake and a blessing’. The young man left in the hope that they might some day resume their ‘lively conversation’, but their paths never crossed again.
That meeting took place in the summer of 1878. Eichling’s memories of his sortie into Moravia were soon overlaid by those from more momentous voyages – until they were exhumed over half a century later, in a world that two wars had changed beyond recognition. And when Eichling told his tale, he instantly won his place in history and was transformed into the object of envy, even jealousy, for a very large number of people around the world.
Augustinian. Newtonian. Darwinian. Mendelian.
Only true giants, those who turn the old order on its head, win the ultimate accolade of having their way of doing things immortalised in their name. St Augustine galvanised Western Christianity with his City of God; Newton’s Principia laid the foundations of modern physics and astronomy; and Darwin rewrote the theory of evolution in The Origin of Species.
But what about Gregor Mendel, the bespectacled monk who fiddled endlessly with peas in his abbey garden? His crossbreeding experiments leap out from the half-remembered pages of our science schoolbooks, a prickly little atoll of algebra in the calm ocean of biology, and his ‘dominant’ and ‘recessive’ genes still stick in our minds – even though none of this has any obvious connection with DNA fingerprinting, designer babies and all the other clever stuff of twenty-first-century molecular genetics. At first sight, Mendel lacks the grandeur of Darwin, Newton or St Augustine.
Johann Mendel was born on 20 July 1822 in Heinzendorf, an agricultural village about 50 kilometres east of Olmütz in Moravia. Johann’s father worked on Peasant Holding No. 58; the local landowner, Countess Maria Waldburga, was judged ‘an enlightened ruler’ but still expected (and got) three days of unpaid servitude from him each week.
Johann was bright enough to catch the eyes of the schoolmaster and priest, who manoeuvred the boy towards the local high school and then the Institute of Philosophy at Olmütz. The cost was difficult for the family to bear, even before the logging accident which left Mendel senior unable to lift a spade. Despite patchy academic performance (and a tendency to return home to lie down for a month or two when it all got too much), Johann was judged a good family investment. His younger sister ploughed her dowry into supporting his studies, and when Holding No. 58 had to be sold because Johann refused to take it over from his father, some of the money was ring-fenced for the lad’s continuing education – on the condition that he trained for the priesthood ‘or some other gainful employment’.
Luckily, he had impressed one of his tutors in Olmiitz, who wrote to commend this ‘very solid character . . . almost first-rate in physics’ to the abbot of the Augustinian Abbey of St Thomas in Brünn. Possibly encouraged by an impending shortage of priests, they snapped him up. On 9 October 1843, the twenty-one-year-old Mendel was admitted as a novice. Johann, the peasant’s son, remained outside the abbey gates; inside, the rechristened Gregor began learning the ways of the Order.
Cyrill Napp, the abbot who welcomed in Mendel, was a short, astute man in his early sixties who ran his abbey with wisdom, wit and a broad mind. Originally an Old Testament and Oriental scholar, Napp was now more interested in heredity and how particular traits are passed on from parent to offspring. His grand aim was to translate scientific theory into agricultural practice: how to boost the wool yield of sheep, or the sweetness of apples, or the quality of the grape harvest. Moravia was a major textile and food producer for the Austrian Empire, and Napp sat strategically, like a spider surveying a network of webs, on powerful local bodies such as the Moravian Agriculture Society and its subgroups for sheep breeders and apple and wine growers. He also went on to co-found the Brünn Natural History Society, which later would acquire fame far beyond its station.
The abbey had begun life as an austere convent, and Napp worked hard to transform it into a mini-university. This seat of learning was remarkably well upholstered, with a no-expense-spared library, magnificent collections of minerals and rare plants, and large gardens which were immaculately conceived and impeccably tended. Creature comforts were not neglected; each friar’s chamber was created by cell fusion (knocking together two or three of the original nuns’ rooms), while the products of the abbey’s kitchens and brewery were the stuff of legend.
Napp hand-picked his fellows for their intellectual promise rather than their devotion to the Scriptures. A photograph from the early 1860s shows the diminutive guru surrounded by his ten friars, all striking poses that convey humour and irony as well as intense cerebral activity (Figure 4.1). They include Pavel Krizkovky, talented composer and troublemaker, who wrote rebel songs and joined anti-monarchist protesters on the streets of Brünn during the People’s Spring of 1848; and Matous Klácel, demoted from teacher to abbey librarian when the higher Church authorities took exception to his ‘radical’ activities. And in the back row is Gregor Mendel, holding one of the products of his labours up to the interested gaze of his companions: not a pea, but a single fuchsia flower.
Figure 4.1 Abbot Cyril Napp (front, centre) with his Brothers at the Abbey of St Thomas, Brünn, probably during the late 1850s.
Big data
Mendel was ordained as a priest in 1847, but Gregor’s metamorphosis was not yet complete. He quickly discovered that the sight of sick people filled him with pathological ‘timidity’; this ruled out any role in the caring professions and by default pushed him into a teaching post at the local Realschule (technical school). His pupils loved him, but this did not prevent him from failing a teaching examination so convincingly that Napp packed him off to Vienna University for a couple of years’ remedial tuition. There, Mendel learned physics, botany and the complexities of ‘combinatorial mathematics’; his teachers were leading experts in their fields and included Christian Doppler, who had recently demonstrated his Effect to an incredulous public with the help of a fast train pulling an open cart containing trumpeters.
This Viennese sojourn set Mendel up to dash off a couple of short papers on garden pests and to fail his teaching examination again, even more spectacularly. Undeterred, Napp welcomed him back to the abbey, and the Realschule – who trusted Mendel’s pupils more than his examiners – appointed him as a Substitute Teacher (Permanent) in classics, science and meteorology. Encouraged by Napp to develop his own research, Mendel began applying statistics to trends in weather patterns and soon became a recognised authority in meteorology. He also dabbled with bees, soil improvement and ornamental plants – hence his spectacular hybrid fuchsias which brought him to the notice of Ernst Benary in the City of Flowers.
The capricious process of plant breeding fascinated Mendel. The offspring of, say, blue and red variants of the same species were not a blue-red blend of purple; instead, they all looked exactly like one of the parents, e.g. blue. However, this blue was more complicated than the seemingly identical true blue of the parent, because if blue offspring were bred with each other, a minority of their progeny turned out red, just like the grandparent that had apparently failed to make its mark on the next generation. Some time in 1854, Mendel decided that fuchsias were too complicated to work out what was happening, and turned instead to Pisum sativum, the garden pea. This was a lucky choice. If instead he had gone for Hieracium (hawkweed), with which he later grappled, the term ‘Mendelian’ would never have entered the scientific lexicon.
And so began an eight-year programme of experiments that generated over 20,000 pea plants and some 300,000 peas, and eventually overflowed from the plot in the abbey garden into a massive greenhouse which Napp ordered to be built for Mendel’s sole use. Mendel first put in two solid years of groundwork to create lines of pea plants that bred true for seven easily recognisable features (traits). Each of these traits occurred in one of two alternate forms, like the faces of a flipped coin. For example, the plant was either tall or short; its peas were either green or yellow; and its flowers were either white or purple. These differences
were clear-cut: the tall plants could look a man in the face, while short ones were under two feet high.
Mendel cross-bred these pure lines, using a fine paintbrush to transfer pollen from the anthers (male organs) of flowers from one form (e.g. tall) to the stigma (female organ) of the alternate form (short, in this case). To prevent nature from interfering, he had already performed keyhole surgery on the recipient flowers, cutting them open and nipping off the anthers while they were still immature and had not yet produced pollen of their own. After fertilising each flower, he tied a tiny calico bag around it to keep out bees and windborne pollen. For completeness, he repeated the experiment in the opposite direction, in this case fertilising the flowers of tall pea plants with pollen collected from short ones.
Within a couple of years, Mendel knew that he had discovered something extraordinary. Even though the seven traits were completely different, all the experiments produced a consistent pattern. Every one of the hybrids looked like one of its parents: all the progeny of a tall x short cross came out tall, while the yellow x green pea cross produced exclusively offspring with yellow peas. But when plants from this first hybrid generation were allowed to fertilise themselves, the characteristic of the missing parent – short plants, or green peas – reappeared in some of the progeny.
Mendel now added the magic touches which allowed him to see what others before him had missed. His experimental numbers were twenty-fold bigger than ever before – over 8,000 plants in the tall x short crosses – which ironed out fluke effects and gave him confidence in the pattern that was emerging. Strikingly, the proportion of plants that resembled the missing parent in the offspring of the self-fertilised hybrid was virtually identical for all seven traits. This proportion was very close to one quarter, so that this version of each trait was outnumbered by the other by a factor of 3 to 1.
Unravelling the Double Helix Page 6