Nature Vol. 291 28 May 1981 285 powerful influence on neurones - recorded (perhaps outside layer IV) contribute to the 1881), equalling the value for 1944. The in other layers - that receive input from the cortical effects of monocular deprivation. next three warmest winters are, in order, 'suppressed' layer. Yet microiontophore­ While the available evidence suggests that 1926, 1958 and I 980. In the past two years tically applied bicuculline would not spread interocular suppression plays a secondary we have, therefore, experienced two far enough to be effective in layers remote role, resolution of the issue must await remarkably warm winters, contrasting from the recording site. Thus Sillito and detailed study of the laminar location and sharply with the winters of 1969 and 1972 colleagues' estimate of the proportion of intracortical connectivity of 'suppressed' which were the two coldest winters since neurones influenced (directly or indirectly) by neurones, as well as attention to potential 1917. For the 1981 winter, especially in GABA-mediated inhibition might well prove inhibitory neurotransmitters other than January and February, the maps published too low. GABA. Plainly, Wiesel and Hubel's by the Free University of Berlin (Berliner In any case, it is evident that both the straightforward observations of eighteen Wetterkarte) show remarkable warm redistribution of atlerents within layer IV years ago opened the door on complex anomalies over northern Canada and and a degree of interocular suppression features of cortical organization. Alaska, and over north-central Siberia - with a much smaller-magnitude cool anomaly over the North Atlantic. Many places in northern Canada and Alaska recorded record warmth in January, some places being more than l6°C above the 1931-60 'normal'. The warmth of the past two winters may Global warming? be reflected in Northern Hemisphere snow cover. In a recent paper in Nature, Matson from T.M.L. Wigley, P.D. Jones and P.M. Kelly and Weisnet 10 described changes in snow cover, based on satellite measurements, IN 1975, Lamb and his colleagues1 noted "variations of air temperature at the surface over the period 1967-1980. Over North that the downward trend of Northern of the Earth may have different signs over America, a rising trend up to 16.9 x 1()6 Hemisphere surface temperatures, which extensive areas of the globe in specific km-2 in 1979 was followed by a sharp drop had begun in the early 1940s, had come to seasons of the year, even against the in 1980(15.1 x 106 km-2). The 1981 value is an end - at least in the northern North background of a significant rise in average even lower, 13.9 x 106 km-2 , the lowest on Atlantic. Since then, the direction of the global characteristics" (p. 97). Thus, while record. For Eurasia the 1980 and 1981 global temperature change has been a an overall warming may have occurred, values are both low (but not as low as the controversial issue. Some argue, quite large regions may still exhibit a 1970 value), while for the whole justifiably, that we cannot be sure of cooling trend. Although the trend revealed hemisphere the values for 1980 and 1981 global-mean conditions because there arc by Borzenkova appears, therefore, to be are 38.9 x J06 km-2 and 38.0 x J06 km-2, such large gaps in the data coverage, well established, one comment5 must be values surpassed only by the 1970 value particularly over the ocean and in the accepted - that we do not know whether (37 .2 x 106 km-2) (1981 data from Michael Southern Hemisphere. However, it can be the warming is liable to continue into the Matson, NOAA/NESS, Washington, DC argued that, because temperatures are future. All of this, of course, begs the 20233). In spite of the correspondence more uniform over the oceans than the questions of what is meant by 'trend', and between recent winter warmth and low continents, large oceanic data gaps can be how one determines the beginning of a snow cover, the overall correlation tolerated (see Barnett2 for an opposing trend, given the large year-to-year between these two variables is low. For the view). Further controversy lies in whether variability. first 13 years of record the correlation or not surface temperatures are meaningful What has happened since 1975? A recent coefficient between hemispheric winter at all. Perhaps a better measure of large­ Russian paper7 gives results up to and snow cover and temperature was -0.03. scale fluctuations in atmospheric tem­ including 1978, and we8 have When 1980 and 1981 are included this rises perature is the column-mean temperature, independently made estimates of Northern to -0.40! as determined, for instance, by the 1 ,000 to Hemisphere average surface temperatures It is difficult to know the significance of 500 mbar height difference. Doubts have up to February 1981. Our own results, these recent events, but it is certainly been expressed by Parker3 as to the based on very much the same data as used premature to associate them with the reliability of these data but, provided the by the Russians, naturally correlate highly effects of increasing atmospheric carbon limitations are taken into account, con­ with theirs. The data show 1972, 1974 and dioxide. At present, the high inter-annual siderable insight into global temperature 1976 to be quite cold years, agreeing with variability of surface temperatures (that is, changes can be gained. the estimates of Barnett2 and Angell and the 'noise' of natural climatic variability) Much useful information can be gained Korshover9 • In fact, the data show 1972 to makes it impossible to identify the from recent Russian publications. have been the coldest year since 1917. apparent trend with any particular causal Borzenkova and his colleagues4 gave a Vinnikov et at. 's data agree for 1972 and mechanism. We note also that a similar series of annual mean temperature values 1976, but they show 1974 as a warm year. A warming trend has occurred before, averaged over the latitude belt 17.5-87.5 cooler 1974 and a cool year in 1976 reduce between the late 1910s and around 1940. 0 N for the period 1881-1975 which show a the magnitude of Borzenkova et at. 's However, a few more consecutive warm marked warming since 1964. Despite 1964-1975 warming trend. However, 1977 winters, with continuing warmth in the criticisms5 , Vinnikov6 has now was a warm year (the warmest since 1953) other seasons, will doubtless give convincingly shown that the data are and both 1979 and 1980 were also relatively climatologists pause for thought. homogeneous and do not suffer from warm. The warming trend appears, I. Lamb, Malmberg & Colebrook Nature 256, 479 ( 1975). discontinuities apparent in earlier work. therefore, to be continuing; but conditions 2. Barnell Mon. Weath. Rev. 106. 1353 (1978). Vinnikov also pointed out that the are still considerably cooler than the balmy 3. Parker Met. Mag. 109, 129 (1980) warming trend they had detected was period between the late 1930s and the early 4. Borzcnkova, Vinnikov, Spirina & Stekhnovskii Meteor ologiya i Gidrologiya 1976(7), 27 ( 1976). certainly not spatially 'universal' ... 1950s. 5. Rubinshtcin MereoroloJ?iya i GidrofoJ?iya 1977(9), 106 Perhaps the most outstanding feature (1977). of recent years has been the 1981 6. Vinnik(n·· Meteorolo!?iya i Gidrologiya 1977(9), 110 ( 1977). T.M.L. Wigley, P.D. Jones and P.M. Kelly are 7. Vinnikov er at. Mereorologiya i Gidrologiya 1980(6), 5 Northern Hemisphere winter (December (1980). in the Climatic Research Unit, School of 8. Jones & Wigley ClimateMonitor9, 43, 86, 109, 147(1980). Environmental Sciences, University of East 1980-February 1981). This was the 9. Angell & Korshover Mon. Weath. Rev. 105.375 (1977). Anglia. warmest winter on record (records began in 10. Matson & Weisnel Nature 289, 451 (1981).

0028-0836/811220285·01$01.00 Cl1981 Macmillan Journals Ltd 286 Nature Vol. 291 28 May 1981 Why mammalian gametes don't mix from J. Michael Bedford

THROUGHOUT the Metazoa the reproduc­ Preliminaries in the female. Incom­ newly ovulated egg (see Fig.!). The tive isolation of a species is consistently patability could first be expressed in the specificity of the immediate preliminaries reflected in the restricted affinities of foreign tract by failure of sperm to be to the union of mammalian gametes is sperm and egg. The possibilities for inter­ transported to the site of fertilization in the difficult to elucidate. At the site of specific fertilization and the precise Fallopian tube, to survive as viable cells, or fertilization, little is known of the nature mechanisms that restrict this among to be capacitated (mammalian sperm are of the elements that promote the acrosome mammals, including man, have been less not able to fertilize immediately after reaction, the role and specificity of action than clear, but an understanding of the ejaculation and must undergo a final of acrosomal enzymes, and the exact basis of this restriction is gradually maturation, capacitation, in the female function of the specific binding between emerging. An ability to dissect or overcome tract). Little is known of sperm function in the reacted sperm head and zona surface. the specific barriers to cross fertilization the native tract, and so the factors affecting Whether once capacitated, spermatozoa could help us come nearer to a precise foreign spermatozoa are hardly defin­ are then triggered to react by a specific analysis of the mechanisms of the normal able. There is no clear pattern to their signal or do so spontaneously is not event. More practically, it may also soon longevity: occasionally it can be prolonged resolved. The response of capacitated offer an avenue for clinical evaluation of beyond that seen in the homologous tract spermatozoa around foreign and native the fertilizing ability and even the chromo­ (rabbit spermatozoa in the estrous rat eggs in the same milieu might illustrate somes of human sperm. Considering the whether the egg coat provides a specific sequential steps of the conception process reaction (as it does in sea urchin) and if allows us to summarize what we know of not, whether the cumulus/ corona cell the factors that prevent easy union of A- -FC--'-- Cel ls of cumulus complex expresses a selective resistance sperm and eggs from different mammals, oophorus towards reacted foreign sperm. That the and why further study of this should be zona pellucida does so is impressively rewarding. illustrated by the many successes in The exact way that a specificity of sperm Oole mma interspecific fusion that follow its removal. and eggs is achieved may vary. Sperm of Periv itelline How it exerts this barrier role still eludes us, teleost fish, for instance, must negotiate space again because the precise mechanism of the specialized environment created homospecific penetration of the zona is around a small hole in the egg shell. largely unexplained. There is an element of Usually, however, organisms practicing 'Catch-22' in that uncapacitated external fertilization seem to depend spermatozoa (which cannot undergo the primarily on elements in the egg coat Fig.l Diagram of a typical mammalian egg acrosome reaction) will often attach to the and sperm surface whose 'fit' dictates soon after ovulation that illustrates the zona pellucida non-specifically in as­ a specificity of penetration; one that for various vestments through which the sociations incompatible with penetration; sympatric marine species prevents the sea sperm must pass before being incorporated into the ooplasm. but in attaining the capacitated state becoming a gamete mixing pot. Since the (in which they undergo the acrosome behavioural preliminaries to ejaculation and uterus), but foreign spermatozoa more reaction), sperm probably undergo a insemination could preserve a specificity often die before they would in a native surface change which imposes specificity in of fertilization Ill mammals, an extreme in­ situation and before native spermatozoa in attachment/binding between the zona dividuality of the character of their gametes the same uterus or oviduct. Nor does the substance and sperm plasmalemma. At all might seem unnecessary. Nonetheless, des­ persistence of motility guarantee transport events it is probable that specific entities, pite changes in the character of both gametes to the site of fertilization. As shown by the possibly glycoproteins, are exposed on the in evolutionary transition from external behaviour of human spermatozoa surface of the capacitated sperm cell fertilization to the rather complex inter­ inseminated into the rabbit, maintenance membrane overlying the acrosome, intact action that characterizes fertilization in of their motility in the vagina is no ready or reacted- and that these bind effectively placental mammals, such an individuality passport through the cervix to the uterine to complementary elements at the surface persists. Cross-species fertilization can cavity. The Fallopian tube likewise tends to of the zona pellucida. In the mouse, one of occasionally follow artificial insemination, select the native male gamete from a mixed three major glycoproteins that form the or the mating that sometimes occurs in con­ population of motile cells placed in the zona pellucida is implicated as the receptor fined situations between related mammals. uterus, and one can only suspect through which specific or functional Further development then depends on the involvement of the specific surface ligands are established. Is the tenacity of harmony of gene expression in the embryo character of the sperm cell at either point of this binding the essential key? The answer and on its intrauterine relationship with the selection in the tract. On the other hand, must await resolution of the means by mother - one favoured possibly by the capacitation does not appear to be an which spermatozoa normally penetrate the superficial rather than the intimate type of obstacle when motility is preserved and zona substance. The thrashing bound placenta. Such embryos often die during spermatozoa can experience the tubal spermatozoon may need the stable union pregnancy (ferret x mink; goat x sheep; rab­ environment. Moreover, the time required of the reacted peri-acrosomal plas­ bit x hare), but others survive to adulthood for capacitation seems more a function of malemma with the zona surface as a as healthy individuals that may be wholly the specific identity of the sperm than of base from which to intrude into the sterile (horse x zebra) or not (ox x bison). In the environment. substance of the zona. It does not seem general, however, mammalian gametes tend Sperm penetration of egg vestments. For good sense, however, that the now­ to express their specific character at several gamete union to occur, sperm must first pass exposed inner membrane of the acrosome points in the process of conception. As a through the substantial barriers around the should bond with receptors throughout the consequence, fertilization will not occur zona, since that surface must oscillate and readily even between members of the same J. Michael Bedford is Uris Foundation slide freely in cleaving a penetration slit. Family such as the rat and mouse, let alone Professor of Reproductive Biology, Cornell Neither of the prime factors being con­ among mammals that differ more widely. University Medical College. sidered - a motive force imparted by the

0028.0836/ 81 / 220286·03$01.00 <1:>1981 Macmillan Journals Ltd