27 JANUARY 2017 • VOL 355 ISSUE 6323 349 SCIENCE sciencemag.org
Such data would improve our understanding of spatial patterns of disturbance and
of how diversity and function recover after
disturbance as well as contribute to models
of whole-ecosystem diversity (15). Remotely
sensed data on tropical forest functional diversity are also useful for monitoring and addressing human impacts on forests in more
meaningful terms than simply the presence
or absence of tree cover alone.
There is a clear and growing need for
incorporating high-quality information
on forest biodiversity and function in decision-making on land use. The scientific
potential of remote-sensing approaches to
meet some of this need is tantalizing. At
least two factors are critical for these approaches to be applied and have impact on
practical decision-making. First, they must
be combined with other ecological work,
both field- and model-based, to address
the most urgent questions for supporting
land-use decision-making. Second, the implications of the patterns and processes
observed for achieving policy goals need to
be communicated clearly and understandably to nontechnical audiences, including
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4. L. Miles, K. Trumper, M. Osti, R. Munroe, C. Santamaria,
2013. REDD+ and the 2020 Aichi Biodiversity Targets
Promoting Synergies in International Forest Conservation
Efforts. Policy Brief #5, UN-REDD Programme; http://bit.
5. L. Miles, V. Kapos, Science 320, 1454 (2008).
6. G. P. Asner et al., Science 355, 385 (2017).
7. T. A. Gardner et al. , Biol. Conserv. 154, 61 (2012).
8. E. A. Law etal. ,Ecol.Appl. 25, 70 (2015).
9. A.M.Guerrero,K.A. Wilson, Conserv. Biol. 10.1111/
10. N. E. Stork et al. , Conserv. Biol . 10.1111/cobi.12883 (2016).
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12. M. Uriarte, R. L. Chazdon,Biotropica48, 915 (2016).
13. L. P. Shoo etal. , Conserv.Lett . 10.1111/conl.12327 (2016).
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By Monique Zetka
Each cell entering the meiotic divisions that ultimately generate eggs and sperm initiates a complex series of events that bring homologous chromo- somes together to ensure their correct subsequent segregation. Anomalies in
this process result in changes to chromosome
number that are detrimental to life. Central
to meiosis in most organisms is the exchange
of DNA sequences between homologous
chromosomes. These recombination events
begin with the formation of programmed
breaks in the DNA and can be repaired to
form the chromosomal crossovers required
for segregation. On pages 403 and 408 of this
issue, Prasada Rao et al. (1) and Ahuja et al.
(2), respectively, point to the unprecedented
involvement of a chromosomally tethered
proteasome in negotiating the transition of
a recombination intermediate into the chromosomal crossovers required for segregation.
DNA double-strand breaks (DSBs) are
catalyzed by a topoisomerase called sporula-tion-specific protein 11 (Spo11) (3). They can
be repaired to generate intermediates that
progress into the crossover pathway and create the necessary linkage between homologs.
This typically occurs in the environment of
the synaptonemal complex, a protein structure that forms between paired chromosomes
(4). DSBs are typically produced in excess of
the number of crossovers that appear, and an
outstanding question is how the crossover-competent precursor is ushered into the
crossover pathway, rather than the competing noncrossover pathway through which the
majority of DSBs will be repaired (5).
The 26S proteasome is a highly conserved
complex of at least 32 subunits that assem-
bles in four to six rings to form the major cel-
lular protease (6). How does the proteasome
recognize a target? A conserved family of E3
ligases catalyzes the addition of small moi-
eties onto proteins, which results in degrada-
tion of the modified targets. The canonical
moiety is the small protein ubiquitin. How-
ever, transfer of the small ubiquitin-related
modifier (SUMO) to a protein target can
provoke a wide number of events, including
the degradation (via SUMO-directed E3 ubiq-
uitin ligases), stabilization, and relocalization
of the target, as well as disruption of the tar-
get’s protein-protein contacts (7).
Prasada Rao et al. and Ahuja et al. provide
strong evidence that proteasome- and SUMO-directed events physically conspire at meiotic chromosomes to promote crossing over.
Both studies show that the proteasome localizes to the meiotic chromosomes of mouse,
budding yeast, and nematode model organisms. This suggests evolutionarily conserved
functions for the complex during meiotic
prophase, when pairs of duplicated chromosomes associate along their entire lengths
and recombine. Prasada Rao et al. show that
in mouse spermatocytes, chemical inhibitors
that disrupt the proteasome, ubiquitylation,
or SUMOylation pathways do not detectably
affect chromosome structure or meiotic DSB
formation, but do disrupt the accumulation
of markers of crossover formation. By deleting the a subunit of the yeast proteasome
core particle or chemically inhibiting the proteasome, Ahuja et al. arrive at the same conclusion as they molecularly monitored the
appearance and kinetics of the DNA strand
invasion events that lead to crossover formation. Prasada Rao et al. also used anti-ubiq-uitin or anti-SUMO antibodies to identify a
robust population of modified proteins at
mouse chromosome axes. Both studies collectively reveal a meiosis-specific chromosomal
requirement for the proteasome and for SUMOylation in the emergence of crossover-competent intermediates that correlate with
final events in recombination.
During meiosis, a conserved family of proteins sharing structural features common
to SUMO E3 ligases has been implicated
as key regulators in the crossover versus
noncrossover decision. In mice, RING finger protein 212 (RNF212) forms numerous
(~150) discrete foci along chromosomes early
in prophase. These foci then reduce to the
few sites of crossover formation (8). Similarly, in budding yeast, localization of ZRT/
IRT-like protein 3 (Zip3) also converges to
crossover sites (9). Although the structures of
RNF212 and Zip3 have implied functions as
Department of Biology, McGill University, Montreal, QC H3A
1B1, Canada. Email: email@example.com
“The results help to elucidate
the inherent functional
variation in a highly
complex ecosystem that
is notoriously difficult to
sample from the ground…”
When degradation spurs
A proteasome controls chromosome pairing and
recombination during gamete formation