Sperm chemoattractants

Raffell JA, Krug PJ, Zimmer RK (2004) The ecological and evolutionary consequences of sperm chemoattraction. Proc Nat Acad Sci USA 101 4501-4506... 

The sperm chemoattractants in nonmammalian species vary to a large extent. Some examples are shown in Table 2. So far, most sperm chemoattractants that have been identified in animals are peptides or low-molecular-weight proteins (l-20kDa), which are heat stable and sensitive to proteases [35, 102]. Exceptions to the rule are the sperm chemoattractants of corals—lipid-hke substances of 140-250 Da [30], and the chemoattractants of the ascidians Ciona—a sulfated steroid [187, 189]. In plants such as ferns, the partially ionized form of malic acid and a large variety of unsaturated four-carbon cis-dicarbojg lic acids are sperm chemoattractants [35], In algae, low-molecular-weight... 

Table 2. Some sperm chemoattractants in nonmammalian speoies.

Al-Anzi, B. and Chandler, D.E. (1998). A sperm chemoattractant is released from Xenopus egg jelly during spawning. Dev. Biol. 198, 366-375. 

CoU, J.C. and Miller, R.L. (1992). The nature of sperm chemoattractants in coral and starfish, in Comparative Spermatology 20 Years After (R. Raccetti, ed.), pp. 129-134. Raven Press, New York. 

Miller, R.D. and Vogt, R. (1996). An N-terminal partial sequence of the 13kDa Pycnopodia helianthoides sperm chemoattractant startrak possesses sperm-attracting activity. y. Exp. Biol. 199, 311-318. 

Olson, J.H., Xiang, X.Y., Ziegert, T, Kittelson, A., Rawls, A., Rieber, A.L. and Chandler, D.E. (2001). AUurin, a 21-kDa sperm chemoattractant from Xenopus egg jelly, is related to mammalian sperm-binding proteins. Proc. Natl. Acad. Sci. U.S.A. 98, 11205-11210. 

Sperm are attracted by chemical substances, which are released by eggs. This process is called chemotaxis and is a well-known phenomenon in most animals and lower plants.4 The attraction of sperm to eggs should counteract the effects of dilution and increase the probability of fertilization in free-swimming invertebrates. Since the fertilization rate may be low under natural conditions, the presence of such chemoattractants is believed to be important. Sperm-attractant substances have been identified in several marine invertebrates, including hydrozoans, mollusks, ascidians, starfish, sea urchins, and urochordates. 

The common denominator of all these mechanisms is that their end result is a directional change up a chemoattractant gradient or down a chemorepellent gradient. These mechanisms should be distinguished from chemokinesis (also termed orthokinesis [7]), which is a mechanism of response that does not involve directional changes and is, therefore, not a part of the broad definition of chemotaxis. In chemokinesis, the linear velocity of the cell or organism is altered by the stimulus [7]. Chemotaxis and chemokinesis may occur in parallel, as in the case of the response of sperm cells to substances secreted from the egg (Chapter 7). 

One of the characteristics of sperm chemotaxis is sperm accumulation at the optimal chemoattractant concentration. However, sperm accumulation can also be caused by speed enhancement (chemokinesis) and trapping. The latter may result from a negative effect of a stimulus on motility, from a gradient-independent change in swimming behavior at a... 

Figure 7. An inverted capillary assay designed to study sperm accumulation in an ascending gradient of a chemoattractant [128]. [Adapted with permission from Eisenbach [48].]...

In this assay, the spermatozoa in the well are suspended in a solution containing the chemoattractant. The capillary, containing either a control buffer or the chemoattractant, is immersed in the sperm suspension. When the capillary contains buffer only, the spermatozoa sense a descending gradient of the chemoattractant as they move from the well to the capillary (Figure 7). When the chemoattractant is in both the capillary and the well, they sense no gradient at all. A comparison is made between these two conditions. This assay thus measures the sperm tendency to leave the chemoattractant rather than to accumulate in it. In the case of sperm chemotaxis, sperm accumulation in the capillary is... 

Generally speaking, there is no single rule with respect to sperm chemotaxis. In some species (for example, in hydroids such as Campan-ularia or tunicate such as Ciona), the swimming direction of the spermatozoa changes abruptly towards the chemoattractant source (Figure 12A). In others (e.g., in hydromedusa, fern, or fish such as Japanese bitterlings), the approach to the chemoattractant source is... 

Figure 11. Effect of the chemoattractant resact on A. punctulata spermatozoa. A mioropipette was inserted into a 20p,l drop of sperm suspension [position of the micropipette is indicated in (a)]. 1 nl of lOnM resact was injected into the drop and 5 s iater the micropipette was removed. Photographs (1s exposures] were taken [a] 5s before insertion of the micropipette, and [b] 2Qs, (c) 40s, (d) 50s, [e] 70s, (f) 90s postinjection. Bar, 200pim. (Taken with permission from Ward eta/. [181].)...

ANP is a polypeptide hormone that is secreted in large quantities by the atrial portion of the heart and from a variety of other mammalian cell types. It exerts many of its actions via activation of particulate guanylate cyclase [16, 134]. ANP is present in human follicular fluids [163] and specific ANP receptors have been identified in human spermatozoa [144]. Sperm chemotaxis to ANP was demonstrated by sperm accumulation in capillaries with ascending [6] and descending [192] gradients and by choice assays [192] (Table 4). It is not yet known whether ANP is involved in sperm chemotaxis in vivo and whether the physiological chemoattractant for human spermatozoa is an ANP-like substance. Since chemotaxis to ANP at physiological concentrations can be observed only in the presence of a neutral endopeptidase inhibitor such... 

In conclusion, follicular fluid, bourgeonal, ANP, and RANTES are the only substances that have been demonstrated to act as chemoattractants for spermatozoa by assays that differentiate between chemotaxis and other processes causing accumulation (Table 1). ANP may not be a direct physiological chemoattractant but rather a guanylate cyclase activator. The significance of sperm chemotaxis to bourgeonal and RANTES is an open question. The identity of the chemoattractant(s) in follicular fluid has yet to be revealed. 

Is sperm chemotaxis specific, i.e., are the chemoattractants specific for each species or are there chemoattractants common to several species There is no single answer to this question. With some exceptions, species specificity appears to be the rule in marine species [102, 103]. There, the gametes are released in to seawater, and gametes of different species may be in close proximity. Therefore, in these cases, chemotaxis may be needed as one of the means to avoid interspecies fertilization. Indeed, in some marine groups (e.g., sea urchins, hydromedusae and certain ophiuroids), the specificity of sperm chemotaxis is very high. In other groups (e.g., starfish), the specificity is at the family level and, within the family, there is no specificity. In contrast, in mollusks, there appears to be no specificity at all ([35,102,103] and references therein). 

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