| ||
台東市八十一歲榮民王忠明，娶妻的隔天就離家出征勦匪去了，結果受傷被俘成了共軍，參加韓戰抗美援朝失敗被送到台灣，再次成為國軍。他怕拖累大陸家人而改名，如今體內還留著勦匪受傷時的彈殼。 昨天，是中國建政六十周年，看著電視直播中共閱兵畫面，對照自己胸前、手臂多處「殺朱拔毛」的反共刺青，走過一甲子的他，有時空錯置的感覺，嘆自己的遭遇是「歷史的錯誤」。 「少小離家卻不能老大回，」王忠明說，即使目前兩岸來往頻繁，回山東老家不難，「我還記得『王宗祿』娶老婆的隔天就離家出征了，家裡的地址都還記得；但人事已非，寄了好幾十封信都沒音信，我是回不去了！」 原名王宗祿的王忠明，十六歲離家征戰，被共軍俘虜，後來派往朝鮮支援韓戰。戰敗後，他選擇追求自由來到台灣，被視為反共義士，同行還有一萬四千多人。 他說為表明反共決心與忠誠，來台前，拿著織麻布袋的大頭針，在身上刺「三民主義、建國救世」「殺朱拔毛、報仇雪恨」等刺青；憂心家人遭追殺，不得不改名，又怕忘了本名，因此在左手刺上本名「王宗祿」。 回憶過往征戰，最慘烈莫過勦匪時中彈，瞬間血流如注。他說：「子彈從左胸穿過左背爆開一個大彈孔，彈頭還在體內，醫生說開刀有危險，一直沒拿，現在身體偶爾會痠痛。」 來台後，他還參加八二三砲戰，常說：「我的生日有很多天，只要是戰後存活的日子都是生日。」五十八年他退伍，擔任公路局司機，在台東展開新生活。 回首過往，他說：「那是一場不得不打的戰爭（國共內戰），多少兄弟當時一個是共軍，一個是國軍，兩相廝殺，沒什麼對錯，就是自保而已。」 |
MATING In many fireflies, pairs stay coupled for hours while the male, lower, gives the female a protein package injected with sperm, called a nuptial gift.
RESEARCHER Sara Lewis is an evolutionary ecologist at Tufts University who studies firefly mating habits.
SPIRALS A coiled nuptial gift may take up a lot of space in a male’s abdomen. Receiving a gift can make a difference in the female’s reproductive success.
LINCOLN, Mass. — Sara Lewis is fluent in firefly. On this night she walks through a farm field in eastern Massachusetts, watching the first fireflies of the evening rise into the air and begin to blink on and off. Dr. Lewis, an evolutionary ecologist at Tufts University, points out six species in this meadow, each with its own pattern of flashes.
Along one edge of the meadow are Photinus greeni, with double pulses separated by three seconds of darkness. Near a stream are Photinus ignitus, with a five-second delay between single pulses. And near a forest are Pyractomena angulata, which make Dr. Lewis’s favorite flash pattern. “It’s like a flickering orange rain,” she said.
The fireflies flashing in the air are all males. Down in the grass, Dr. Lewis points out, females are sitting and observing. They look for flash patterns of males of their own species, and sometimes they respond with a single flash of their own, always at a precise interval after the male’s. Dr. Lewis takes out a penlight and clicks it twice, in perfect Photinus greeni. A female Photinus greeni flashes back.
“Most people don’t realize there’s this call and response going on,” Dr. Lewis said. “But it’s very, very easy to talk to fireflies.”
For Dr. Lewis, this meadow is the stage for an invertebrate melodrama, full of passion and yearning, of courtship duets and competitions for affection, of cruel deception and gruesome death. For the past 16 years, Dr. Lewis has been coming to this field to decipher the evolutionary forces at play in this production, as fireflies have struggled to survive and spread their genes to the next generation.
It was on a night much like this one in 1980 when Dr. Lewis first came under the spell of fireflies. She was in graduate school at Duke University, studying coral reef fish. Waiting for a grant to come through for a trip to Belize, she did not have much else to do but sit in her backyard in North Carolina.
“Every evening there was this incredible display of fireflies,” Dr. Lewis said. She eventually started to explore the yard, inspecting the males and females. “What really struck me was that in this one-acre area there were hundreds of males and I could only find two or three females,” she said. “I thought, ‘Man, this is so intense.’ ”
When a lot of males are competing for the chance to mate with females, a species experiences a special kind of evolution. If males have certain traits that make them attractive to females, they will mate more than other males. And that preference may mean that those attractive males can pass down their traits to the next generation. Over thousands of generations, the entire species may be transformed.
Charles Darwin described this process, which he called sexual selection, in 1871, using male displays of antlers and feathers as examples. He did not mention fireflies. In fact, fireflies remained fairly mysterious for another century. It was not until the 1960s that James Lloyd, a University of Florida biologist, deciphered the call and response of several species of North American firefly.
Dr. Lewis, realizing that other firefly mysteries remained to be solved, switched to fireflies from fish in 1984, when she became a postdoctoral researcher at Harvard. She taught herself Dr. Lloyd’s firefly code and then began to investigate firefly mating habits. North American fireflies spend two years underground as larvae, then spend the final two weeks of their lives as adults, flashing, mating and laying eggs. When Dr. Lewis started studying fireflies, scientists could not say whether the females mated once and then laid all their eggs, or mated with many males. “Nobody knew what happened after the lights went out,” Dr. Lewis said.
She searched for mating fireflies in the evening, marked their locations with surveyor’s flags and then revisited them every half-hour through the night. They were still mating at dawn.
“It was cool to watch the sun rise and see the couples breaking up and the females crawling down the grass to lay their eggs,” Dr. Lewis said.
Many Americans are familiar with the kinds of fireflies Dr. Lewis studies, but they represent only a tiny fraction of the 2,000 species worldwide. And there is enormous variation in these insects. “There are some species that produce flashes when they’re adults, and there are some that simply glow as adults,” Dr. Lewis said. “Then there are a whole bunch of species where the adults don’t produce any light at all.”
In recent years scientists have analyzed the DNA of fireflies to figure out how their light has evolved. The common ancestor of today’s fireflies probably produced light only when they were larvae. All firefly larvae still glow today, as a warning to would-be predators. The larvae produce bitter chemicals that make them an unpleasant meal.
As adults, the earliest fireflies probably communicated with chemical signals, the way some firefly species do today. Only much later did some firefly species gain through evolution the ability to make light as adults. Instead of a warning, the light became a mating call. (An enzyme in the firefly’s tail drives a chemical reaction that makes light.)
The more Dr. Lewis watched firefly courtship, the clearer it became that the females were carefully choosing mates. They start dialogues with up to 10 males in a single evening and can keep several conversations going at once. But a female mates with only one male, typically the one she has responded to the most.
Dr. Lewis wondered if the female fireflies were picking their mates based on variations in the flashes of the males. To test that possibility, she took female fireflies to her lab, where she has computer-controlled light systems that can mimic firefly flashes. “You can play back specific signals to females and see what they respond to,” Dr. Lewis said.
The female fireflies turned out to be remarkably picky. In many cases, a male flash got no response at all. In some species, females preferred faster pulse rates. In others, the females preferred males that made long-lasting pulses.
If females preferred some flashes over others, Dr. Lewis wondered why those preferences had evolved in the first place. One possible explanation was that the signals gave female fireflies a valuable clue about the males. Somehow, mating with males with certain flash patterns allowed females to produce more offspring, which would inherit their preference.
It is possible that females use flashes to figure out which males can offer the best gifts. In many invertebrate species, the males provide females with food to help nourish their eggs. Dr. Lewis and her colleagues discovered that fireflies also made these so-called nuptial gifts — packages of protein they inject with their sperm.
Dr. Lewis is not sure why she and her colleagues were the first to find them. The gifts form coils that can take up a lot of space in a male firefly’s abdomen. “They’re incredibly beautiful,” she said.
Receiving nuptial gifts, Dr. Lewis and her colleagues have shown, can make a huge difference in the reproductive success of a female firefly. “It just about doubles the number of eggs a female can lay in her lifetime,” she said. One reason the effect is so big is that fireflies do not eat during their two-week adulthood. A slowly starving female can use a nuptial gift to build more eggs.
In at least some species, females may use flashes to pick out males with the biggest gifts. Dr. Lewis has tested this hypothesis in two species; in one, males with conspicuous flashes have bigger gifts. In another species, she found no link.
“In some cases they could be honest signals, and in some cases they could be deceptive signals,” Dr. Lewis said.
Deception may, in fact, evolve very easily among fireflies. It turns out that a male firefly does not need to burn many extra calories to make flashes. “It takes some energy, but it’s tiny. It’s less costly for a male than flying around,” Dr. Lewis said.
If making light is so cheap for males, it seems odd that they have not all evolved to be more attractive to females. “What is it that keeps their flashes from getting longer and longer or faster and faster?” Dr. Lewis asked.
Scanning the meadow, she grabbed her insect net and ran after a fast-flying firefly with a triple flash. She caught an animal that may offer the answer to her question. Dr. Lewis dropped the insect into a tube and switched on a headlamp to show her catch. Called Photuris, it is a firefly that eats other fireflies.
“They are really nasty predators,” Dr. Lewis said. Photuris fireflies sometimes stage aerial assaults, picking out other species by their flashes and swooping down to attack. In other cases, they sit on a blade of grass, responding to male fireflies with deceptive flashes. When the males approach, Photuris grabs them.
“They pounce, they bite, they suck blood — all the gory stuff,” Dr. Lewis said. She has found that each Photuris can eat several fireflies in a night. Photuris kills other fireflies only to retrieve bad-tasting chemicals from their bodies, which it uses to protect itself from predators.
To study how Photuris predation affects its firefly prey, Dr. Lewis and her colleagues built sticky traps equipped with lights that mimicked courtship signals of Photuris’s victims. The scientists found that Photuris was more likely to attack when flash rates were faster. In other words, conspicuous flashes — the ones females prefer — also make males more likely to be killed.
“At least where Photuris predators are around,” Dr. Lewis said, “there’s going to be a strong selection for less conspicuous flashes.”http://www.cbs.dtu.dk/staff/dave/roanoke/genetics980218.html
Biology 210
GENETICS
18 February, 1998
6.4b The Nucleosome is the Basic Structure Unit in Chromatin.
6.4c Nucleosome Core Particles.
6.4d The Arrangement of Chromatin Fibers in a Chromosome.
6.4e DNA sequences affecting chromosome condensation.
6.5 Polytene Chromosomes.
 | Just like in bacteria, the DNA in eukaryotes is highly compacted (roughly 7000x in mitotic chromosomes). However, unlike bacteria, in most eukaryotes, the DNA forms stable protein complexes. Here's a picture of a "lampbrush chromosome", which is thought to reflect the underlying orgainsation of all chromosomes, with a central scaffold (here stained brightly) and projecting lateral loops (stained red) formed by a folded continuous strand of DNA associated with histone proteins. |
If you were to extract the DNA from a single, linear chromosome, and stretch it out, it would be one very long molecule (more than 2 cm for human chromosome 1). |  |
The level of chromosome condensation can be monitored using electron microscopy. It is possible to control the condensation by varying the ionic strength of the solution.
Here is a very famous picture in which the chromosome (see the "X" in the middle) has been carefully manipulated such that all of the histone proteins have been removed - you can see all the DNA make loops to and from the central scaffold. There is an enormous amount of DNA in the chromosomes. |  |
The first level of compaction is where the DNA wraps around nucleosomes.
On the Histone family of proteins:
There are 4 histone components of the nucleosome:
| histone | evolutionary conservation |
| H2A | moderately conserved |
| H2B | moderately conserved |
| H3 | very conserved |
| H4 | nearly 100% conserved |
Most organisms contain many copies of the histone genes, in tandem repeats scattered through the chromosomes.
The organisation of nucleosomes. The DNA molecule is wrapped around the nucleosome about 2 times. The nucleosome actually consists of a histone octamer, with two copies each of histones H2A, H2B, H3, and H4. This is Figure 6.8 from you text (page 231).
Figure 6_10 from Hartl & Jones. The DNA is wrapped around the nucleosome core particles, which are then condensed into a 30 nm fiber. This then folds into a larger 300 nm fiber, which coils up into a 700 nm fiber, which then makes up the 1400 nm chromatid arm.
This 30 nm fiber is then further compacted:
The chromosome condensation is a dynamic process, as can be seen by the following drawing of chromosomes in meiotic prophase from a protozoan:
Remember from last week, there are certain DNA sequences that can be quite rigid, and some sequences can be flexible. There are some DNA sequences which can facilitate chromatin condensation, and amplification of these sequences results in large regions of chromatin that is very condensed. One such motif is the CGG triplet repeat, which, when amplified causes the DNA in the chromosome to form what is called a "fragile site", because when viewed under the electron microscope, this region looks like it might break off quite easily. In fact, chromosome breakage at this point DOES occur quite often, and it is associated with many different types of genetic diseases in humans. For example, "fragile X" syndrome is the most common genetic form of mental retardation in humans - and the molecular basis of this disease has been found to be due to the amplification of a triplet repeat of the CGG sequence from about 20 copies (e.g., 60 bp long) to a repeat of up to several THOUSAND base pairs in length. When the DNA gets amplified, the resulting change in chromosome structure provides a "fragile site". Amplification of triplet repeats is also responsible for more than 40 other diseases in humans, including Huntington disease.
 J Biol Chem 1996 Oct 4;271(40):24325-24328 Nucleosome assembly on methylated CGG triplet repeats in the fragile X mental retardation gene 1 promoter. Godde JS, Kass SU, Hirst MC, Wolffe AP Laboratory of Molecular Embryology, NICHHD, National Institutes of Health, Bethesda, Maryland 20892-5430, USA. Expansion and methylation of CGG repeat sequences is associated with Fragile X syndrome in humans. We have examined the consequences of CGG repeat expansion and methylation for nucleosome assembly and positioning on the Fragile X Mental Retardation gene 1 (FMR1) gene. Short unmethylated CGG repeats are not particularly favored in terms of affinity for the histone octamer or for positioning of the reconstituted nucleosome. However, upon methylation their affinity for the histone octamer increases and a highly positioned nucleosome assembles with the PMID: 8798682, UI: 96394576 |
Last modified on: 2 February, 2000 by Dave Ussery
http://www.cbs.dtu.dk/staff/dave/roanoke/genetics980218.html
Biology 210
GENETICS
18 February, 1998
6.4b The Nucleosome is the Basic Structure Unit in Chromatin.
6.4c Nucleosome Core Particles.
6.4d The Arrangement of Chromatin Fibers in a Chromosome.
6.4e DNA sequences affecting chromosome condensation.
6.5 Polytene Chromosomes.
| | Just like in bacteria, the DNA in eukaryotes is highly compacted (roughly 7000x in mitotic chromosomes). However, unlike bacteria, in most eukaryotes, the DNA forms stable protein complexes. Here's a picture of a "lampbrush chromosome", which is thought to reflect the underlying orgainsation of all chromosomes, with a central scaffold (here stained brightly) and projecting lateral loops (stained red) formed by a folded continuous strand of DNA associated with histone proteins. |
If you were to extract the DNA from a single, linear chromosome, and stretch it out, it would be one very long molecule (more than 2 cm for human chromosome 1). | |
The level of chromosome condensation can be monitored using electron microscopy. It is possible to control the condensation by varying the ionic strength of the solution.
Here is a very famous picture in which the chromosome (see the "X" in the middle) has been carefully manipulated such that all of the histone proteins have been removed - you can see all the DNA make loops to and from the central scaffold. There is an enormous amount of DNA in the chromosomes. | |
The first level of compaction is where the DNA wraps around nucleosomes.
On the Histone family of proteins:
There are 4 histone components of the nucleosome:
| histone | evolutionary conservation |
| H2A | moderately conserved |
| H2B | moderately conserved |
| H3 | very conserved |
| H4 | nearly 100% conserved |
Most organisms contain many copies of the histone genes, in tandem repeats scattered through the chromosomes.
The organisation of nucleosomes. The DNA molecule is wrapped around the nucleosome about 2 times. The nucleosome actually consists of a histone octamer, with two copies each of histones H2A, H2B, H3, and H4. This is Figure 6.8 from you text (page 231).
Figure 6_10 from Hartl & Jones. The DNA is wrapped around the nucleosome core particles, which are then condensed into a 30 nm fiber. This then folds into a larger 300 nm fiber, which coils up into a 700 nm fiber, which then makes up the 1400 nm chromatid arm.
This 30 nm fiber is then further compacted:
The chromosome condensation is a dynamic process, as can be seen by the following drawing of chromosomes in meiotic prophase from a protozoan:
Remember from last week, there are certain DNA sequences that can be quite rigid, and some sequences can be flexible. There are some DNA sequences which can facilitate chromatin condensation, and amplification of these sequences results in large regions of chromatin that is very condensed. One such motif is the CGG triplet repeat, which, when amplified causes the DNA in the chromosome to form what is called a "fragile site", because when viewed under the electron microscope, this region looks like it might break off quite easily. In fact, chromosome breakage at this point DOES occur quite often, and it is associated with many different types of genetic diseases in humans. For example, "fragile X" syndrome is the most common genetic form of mental retardation in humans - and the molecular basis of this disease has been found to be due to the amplification of a triplet repeat of the CGG sequence from about 20 copies (e.g., 60 bp long) to a repeat of up to several THOUSAND base pairs in length. When the DNA gets amplified, the resulting change in chromosome structure provides a "fragile site". Amplification of triplet repeats is also responsible for more than 40 other diseases in humans, including Huntington disease.
| J Biol Chem 1996 Oct 4;271(40):24325-24328 Nucleosome assembly on methylated CGG triplet repeats in the fragile X mental retardation gene 1 promoter. Godde JS, Kass SU, Hirst MC, Wolffe AP Laboratory of Molecular Embryology, NICHHD, National Institutes of Health, Bethesda, Maryland 20892-5430, USA. Expansion and methylation of CGG repeat sequences is associated with Fragile X syndrome in humans. We have examined the consequences of CGG repeat expansion and methylation for nucleosome assembly and positioning on the Fragile X Mental Retardation gene 1 (FMR1) gene. Short unmethylated CGG repeats are not particularly favored in terms of affinity for the histone octamer or for positioning of the reconstituted nucleosome. However, upon methylation their affinity for the histone octamer increases and a highly positioned nucleosome assembles with the PMID: 8798682, UI: 96394576 |
Last modified on: 2 February, 2000 by Dave Ussery
