CREDIT: NASA/JPL
PERSPECTIVES
grouped according to geographical location
or ethnicity. They used a program called
STRUCTURE to cluster these sequences into
a user-defined number of populations,
based on differences in the frequencies of
nucleotides at 1418 polymorphic sites. They
obtained their most consistent results with
five different H. pylori populations, which
correspond well to the human populations
from which the isolates were recovered.
They then applied an algorithm that allows
runs of polymorphic nucleotides to be derived from any of the five populations, and
an iterative process that progressively refines the assignment of individual polymorphisms to the five populations, by taking account of the population assignments of the
neighboring polymorphisms.
In some H. pylori isolates, all polymorphisms along each gene segment are assigned
with confidence to a single human population. However, there are clear examples of
population mosaics, where a small run of
polymorphisms in a gene segment, or a
whole gene segment, appears to have been introduced by a recombinational replacement
from a coinfecting H. pylori derived from a
different population (see the figure). The extent and nature of the population mosaicism
in H. pylori genes are consistent with our current knowledge of human migrations and
population mixing. For example, H. pylori
from Maoris, and those from isolated
Amerindian populations, are very cleanly assigned as East Asian in origin, supporting the
view that H. pylori was first introduced into
the New World by ancient human migrations
(6), rather than by Europeans. However, isolates from African Americans or South
African Asians often show their ancestral origins, with superimposed replacements from
populations in their adopted country and, as
expected, H. pylori genes from Europeans are
complex mosaics (see the figure).
The origins of human populations and
their migratory routes exert a continuing fascination. Migrations have been tracked by
studying the genetic similarities and differences among modern populations, or by using
some proxy measure of genetic similarity—
typically linguistic “phylogeny” (7). Can the
large number of polymorphisms present in H.
pylori populations provide another useful
proxy that can resolve controversial aspects of
human population movements? Probably not.
Falush et al. show that H. pylori sequences are
surprisingly good indicators of human migrations where relocating groups remain isolated,
but acquisition of H. pylori sequences from
other populations becomes problematic when
populations intermingle. Untangling the complexities of population migrations in Europe
or Asia, and the confounding effects of interbreeding, requires direct rather than proxy
measures of human ancestry. However, the
approaches used by Falush et al. can be extended to humans; a paper using STRUCTURE
to identify human populations recently appeared in this journal (8). The real excitement
would be to use large numbers of single-nucleotide polymorphisms (SNPs) to visualize
directly the population mosaicism along human chromosomes, and to link this to known
ancestries, which in some families extend
back well over a thousand years.
The Falush et al. strategy also could be
applied to study the levels of social interaction required for the acquisition of H. pylori. For example, analysis of the H. pylori
sequences acquired by children born to immigrant families, in relation to the extent of
their social interactions with the local community (use of nannies, day-care attendance,
and so forth), could refine our ideas about
routes of transmission. Understanding population mosaicism in H. pylori genes in
terms of family history and social history
could also be fascinating.
Infection with H. pylori has important
health consequences as it causes duodenal
ulcers and increases the risk of gastric cancers. H. pylori populations are genetically diverse, and high rates of recombination, and
transmission from mother to infant, will result in large differences among ethnic groups
in the frequencies of alleles at loci associated with progression to disease. As pointed
out by Falush et al., these differences may
explain the substantial variation in rates of
gastric cancer per infected individual in different human populations, although other
factors are undoubtedly important.
References
1. D. J. Funk, L. Helbling, J. J. Wernegreen, N. A. Moran,
Proc. R. Soc. London Ser. B 267, 2517 (2000).
2. D. Falush et al., Science 299, 1582 (2003).
3. D. Rothenbacher et al., J. Infect. Dis. 179, 398 (1999).
4. Y. Tindberg et al., Gastroenterology 121, 310 (2001).
5. D. Falush et al., Proc. Natl. Acad. Sci. U.S.A. 98, 15056
(2001).
6. C. Ghose et al., Proc. Natl. Acad. Sci. U.S.A. 99, 15107
(2002).
7. L. L. Cavalli-Sforza, Genes, Peoples and Languages
(Penguin Books, London, 2001).
8. N. A. Rosenberg et al., Science 298, 2381 (2002).
P L A N E TA R Y S C I E N C E
Cassini Imaging at Jupiter
Larry Esposito
n 30 December 2000, the Cassini
spacecraft made its closest flyby to
the planet Jupiter on its way to
Saturn. For 6 months, Cassini’s state-of-theart cameras and other experiments watched
the behavior of Jupiter, its magnetosphere,
moons, and rings. The cameras provided an
unsurpassed data set on atmospheric motions and other phenomena, reported by
Porco et al. on page 1541 of this issue (1).
Cassini discovered surprising new phenomena and provided key information about
Jupiter’s meteorology, rings, and moons.
Cassini, a joint mission of NASA, ESA,
and ASI (the Italian Space Agency), is the
most ambitious planetary space mission
ever launched. The spacecraft (see the first
figure) is nearly 7 m in length, carrying 12
experiments on its orbiter and six more on
the European Huygens probe, which will
land on Saturn’s giant moon Titan. After
launch on 15 October 1997, Cassini flew
twice past the planet Venus, made a close
approach to Earth, and then flew past
Jupiter on the way to Saturn, where it will
begin orbiting on 1 July 2004. The Huygens
probe will descend through Titan’s atmosphere, directly sampling its composition
before landing on 14 January 2005. On
O
Titan’s cold surface, the probe will hopefully survive for more than 30 min, sending
pictures and analysis of the surface back to
Earth, using the orbiter as a radio relay link.
In December 2000 and January 2001, both
the Cassini spacecraft and the NASA Galileo
orbiter were in the close vicinity of Jupiter,
The author is in the Laboratory for Atmospheric and
Space Physics, University of Colorado, Boulder, CO
80309, USA. E-mail: larry.esposito@lasp.colorado.edu
Cassini spacecraft awaits launch at Kennedy
Space Center. Cassini is the most ambitious
planetary mission to date.
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SCIENCE
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7 MARCH 2003
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PERSPECTIVES
providing the first-ever conjunction of two
spacecraft at an outer planet. Nearly simultaneous measurements were also made by the
Hubble Space Telescope and the Chandra Xray Observatory from Earth orbit. Because
Galileo’s high-gain antenna failed to open, the
spacecraft has very limited communication to
Earth and cannot provide the larger picture of
Jupiter meteorology. Cassini scientists and engineers took this opportunity to take simultaneous measurements with the Galileo spacecraft, to provide a long continuous look at the
Jupiter system, and to test their experiments in
1979 flyby) and Galileo. But Cassini spectacularly succeeded in providing 6 months
of global, continuous viewing of Jupiter’s
atmosphere.
It is too soon to say whether these data
can answer the question of the ultimate
source of the bands and eddies on Jupiter
(see the second figure). Do these arise from
small convective storms gradually aggregating into the large, organized motion? Do
the larger storms thus “feed” on this energy
source to sustain their long existence? The
“Great Red Spot” is a centuries-old hurricane that could hold several Earths.
An active atmosphere. The Cassini images show it gobbling
Jupiter’s atmosphere has a up several smaller storms (1), supportbanded appearance with ing this scenario.
Cassini’s observations of Jupiter’s
many atmospheric phenomena, including the Great Red polar region have been assembled inSpot seen on the lower right. to a movie that shows surprising new
Cassini’s flyby in late 2000 phenomena. Toward the poles, Jupiter’s
provided global movies of banded appearance fades, and hunthe planet’s meteorology.
dreds of interacting vortices are seen.
Small-scale features north of 60° latipreparation for the 4-year tour of the Saturn tude grow and disappear in a period of
system. The first Cassini imaging results are weeks. A large dark oval—as big as the
presented by Porco et al. in this issue (1). Great Red Spot—grew, developed a bright
Cassini measurements of the Jupiter radiation core, began to circulate clockwise, and fienvironment, which complement the imaging nally elongated and thinned, gradually
results reported here, have been published disappearing. This storm may have been
previously (2).
triggered by an event in Jupiter’s magneDuring the Jupiter flyby, the Cassini tosphere: Its location coincides with the
camera system collected 26,000 images be- region where particles from Jupiter’s raditween 1 October 2000 and 22 March 2001. ation belts enter the atmosphere (3), causThe main purpose of the flyby was to ac- ing bright aurorae (like the northern lights
celerate the spacecraft on to Saturn. At the on Earth). Cassini is now planning comclosest approach of 9.72 million km (136 parable observations of Saturn’s polar retimes Jupiter’s radius), the images have a gions to seek similar phenomena there.
resolution of 58 km, not as good as the best
The Cassini cameras observed aurorae
images sent back by Voyager (during its on the back side of Jupiter while simultane-
ous measurements were made by Hubble
from Earth orbit. These data confirm that
the auroral region is larger on the night side,
as expected from variation in the pressure of
the solar wind. The moons Io and Europa
were photographed when eclipsed from the
Sun by Jupiter, showing visible glows from
electrons that strike their thin atmospheres.
These observations will be fruitfully compared with those from Hubble to better characterize this atmospheric phenomenon (4).
Movies of Jupiter’s very faint and thin
rings confirm that small moons like Metis
and Adrastea are the immediate source of
the ring particles. The meteoritic bombardment of these objects knocks off dust particles that then form the visible ring around
Jupiter. Porco et al. make good use of the
particular angles at which Cassini observed
to argue that the ring particles are not
spherical, as was previously assumed.
The Cassini Jupiter flyby was a great success, helping to prepare for the Cassini Saturn
mission and providing key data sets (including images and movies) about the meteorology of Jupiter, its moons, magnetosphere, and
ring system. Saturn has only been visited
briefly by Pioneer (1979) and the two Voyager
spacecrafts (1980, 1981). The planned 4-year
orbital mission will allow long-term studies
and follow-up observations of new discoveries. The Jupiter results provide some hints of
the spectacular new findings that await
Cassini when it reaches Saturn.
References
1.
2.
3.
4.
C. C. Porco et al., Science 299, 1541 (2003).
T. W. Hill, Nature 415, 965 (2002).
J. T. Clarke et al., Nature 415, 997 (2002).
M. A. McGrath et al., Bull. Am. Astron. Soc., DPS meeting abstract 34.09 (2000).
State Transitions—a Question
of Balance
John F. Allen
reen plants and algae use a process
of photochemical energy transduction called photosynthesis to harness
light energy to make the energy-rich molecule ATP. Within their chloroplasts, light energy captured by chlorophyll photopigments
is transformed into an electrochemical potential, which raises the energy of an electron; the subsequent “fall” of the electron
G
The author is in the Department of Plant
Biochemistry, Center for Chemistry and Chemical
Engineering, Box 124, Lund University, SE-221 00
Lund, Sweden. E-mail: john.allen@plantbio.lu.se
1530
back to its original state releases energy that
is used to make ATP. Plants must tune photosynthesis to changing light conditions,
and they do this with kinases that phosphorylate (add phosphate groups) to proteins of
the photosynthetic machinery. The lightharvesting complex II (LHCII) is found in
the chloroplasts of all plants and green algae, and accounts for about half of the
chlorophyll molecules in nature. It tunes energy conversion to the wavelength of light in
a balancing act known as state transitions.
For over 20 years, the redox-controlled kinase that phosphorylates proteins in the
7 MARCH 2003
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SCIENCE
LHCII and thus drives state transitions has
been eagerly sought. Despite ingenious biochemical experiments, the results have invariably been ambiguous, yielding interesting new proteins but leaving the identity of
the LHCII kinase shrouded in mystery (1).
Enter Depège et al. (2) on page 1572 of this
issue, with their report of a new LHCII kinase. Using a genetic approach to screen for
mutants of the green alga Chlamydomonas
reinhardtii, they identify a new serine-threonine protein kinase in the chloroplast thylakoid membranes. They call their kinase
Stt7 (for state transition, thylakoid) and
demonstrate that it is required for the phosphorylation of the LHCII protein complex.
Both light and dark reactions comprise
the energy conversion steps of photosynthesis. During the former, light energy drives
the movement of an electron from a reluctant
donor to a reluctant acceptor. This is followed by dark reactions during which the
electron is returned to its lowest energy state
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CREDIT: NASA/JPL
B O TA N Y